Formed in Gunma in 2004, with Iyori Shimizu as the central figure, the band went through several member changes before reaching its current lineup in 2007.

On February 18th, 2009, they released their first mini album, "逃した魚" (The Fish That Got Lost). For the first time in history, they were simultaneously selected as Tower Records' "Tower Recommend" and HMV's "HOT PICKS," leading to an unprecedented large-scale distribution in stores nationwide. In June 2010, they released their long-awaited first full album, "あとのまつり" (The Festival After). They established themselves as a band with heartbreaking lyrics and beautiful melodies that set them apart from the rest.

Just before their debut, they were selected by iTunes as the new artist most likely to break out in 2011, "Japan Sound of 2011." Their major debut single, "はなびら," released on April 6th, reached number 45 on the Oricon Weekly Chart, and their second single, "花束," released on June 22nd, received 73 hits, including ending plays on FM radio stations nationwide and as the ending theme for TV programs, reaching number 18 on the Oricon Weekly Chart. They then released their third single, "思い出せなくなるその日まで" in October, followed by their first major album, "スーパースター" which reached number 4 "スーパースター" the Oricon Weekly Chart.

In 2011, their second single "花束" reached number one on the annual radio airplay chart for Japanese music, won the Gold Disc Award for Best 5 New Artist, and was awarded the CD Shop Award. Their solo tour of 11 locations across the country (including two additional shows) was sold out immediately, and their additional show at Shibuya Public Hall was also sold out immediately. Despite being a newcomer, they have also performed at six summer festivals: "ROCK IN JAPAN," "RISING SUN ROCK FES.", "MONSTER baSH," "SETSTOCK," "HIGHER GROUND," and "TREASURE05X."

In 2012, they released four singles: "恋," "日曜日," "わたがし," and "青い春." "日曜日" was used as the theme song for the HBC/TBS drama "Soup Curry," and "青い春" was used as the theme song for the CX drama "Koukou Nyushi," attracting even more attention. In November, they released their third album, "blues" which included these singles, and then embarked on a nationwide tour with 19 shows in 17 locations, all of which sold out on the day of their release. Furthermore, their first performance at the Nippon Budokan on September 7, 2013, was also a huge success, selling out on the day of their release.

In 2014, they released their new album "Love Story" on March 26th, which includes their 8th single "高嶺の花子さん" (released in June 2013), 9th single "fish"(released in February 2014) and 10th single "繋いだ手から" (released in March 2014), and has already sold over 120,000 copies!!! In May, they embarked on their first nationwide hall tour "Love Stories Tour 2014" (20 locations, 21 shows) to promote the album, all of which sold out on the same day!!! The additional shows, a 2-day concert at Yokohama Arena and a 1-day concert at Osaka-jo Hall, were also sold out on the same day, making for a huge success.

Their single "ヒロイン," released in January 2015, was used as the commercial song for JR SKISKI, "SISTER," released in May, was the commercial song for Otsuka Pharmaceutical's Pocari Sweat Ion Water, "手紙," released in August, was the commercial song for NTT Docomo, and "クリスマスソング," released in November, was used as the theme song for the Fuji TV Monday 9pm drama "5→9~My Buddhist Boyfriend~," both of which became huge hits. Their fifth original album, "シャンデリア," then topped the Oricon Weekly Chart for two consecutive weeks.

In January 2016 they, embarked on a hall tour of 27 shows at 26 locations, followed by an arena tour of 12 shows at 6 locations, all of which sold out immediately. Their single "僕の名前を" released in May, was the theme song for the film "Wolf Girl and Black Prince," reaching number one on the iTunes weekly chart, and his single "ハッピーエンド," released in November, attracted attention as the theme song for the film "I'll Date Tomorrow, with Yesterday's You." They then released their first greatest hits album, "アンコール" on December 28th, becoming a huge hit, selling over 800,000 copies.

Their largest nationwide arena tour to date, "All Our Yesterdays Tour 2017," which began in February 2017, attracted 300,000 people, and their song "瞬き," the theme song for the film "The 8-Year Engagement: A True Story of a Miracle," released in December 2017, became a best-seller. From July to October, they completed a dome tour with five shows in Tokyo, Nagoya, and Osaka, all of which were sold out, resulting in great success. Following the success of "オールドファッション," the theme song for the TBS Friday drama "Big Love: With You Who Forgets Me" in October 2018, and "HAPPY BIRTHDAY" the theme song for the TBS Tuesday drama "A Story to Read on the Day You Fall in Love for the First Time" in January 2019, their sixth original album "MAGIC" became a huge hit, selling over 500,000 copies, and their "NO MAGIC TOUR 2019," which began in April 2019, was a huge success, attracting 350,000 people across 35 performances.
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In August 2020, "水平線" (Horizon), a song written for high school students who had aimed to participate in the Inter-High School Championships that were canceled due to the COVID-19 pandemic, was chosen as the theme song for the canceled "Reiwa 2nd Year Inter-High School Championships," and became a big topic of conversation as it spread widely on social media, mainly among teenagers.
In October, they released "エメラルド" the theme song for the TBS Sunday drama "Kiken na Venus," as a digital single.
In 2021, back number took on the role of songwriter and producer for BTS's new song "Film out" marking their first time providing music for an artist. This garnered attention not only in Japan but also around the world.
"怪盗," which was released digitally in April, was used as the theme song for the NTV Wednesday drama "Koi wa Deep ni" and became a huge hit.
In August, "水平線" (Horizon), which had been released exclusively on YouTube since the previous year, was released digitally on August 13th, the opening day of the 2021 Inter-High School Championships.
In September, they released their 21st single, "黄色" which became the theme song for the ABEMA original romance program "Niji to Ookami ni wa Damasarenai" (Don't Be Fooled by Rainbows and Wolf).
In August 2022, they digitally released "ベルベットの詩" the theme song for the movie "Akira to Akira." In October, they digitally released "I Love You," the theme song for the NHK serial drama "アイラブユー!".

On January 17, 2023, they released their 7th album, "ユーモア" their first in approximately four years.
back number 's first-ever five-dome tour, "in your humor tour 2023," which began in March and included Sapporo, Tokyo, Aichi, Osaka, and Fukuoka, was a huge success, selling out immediately.
In August, they released "怪獣のサイズ" digitally. In October, they released the Live Blu-ray & DVD "in your humor tour 2023 at Tokyo Dome".

In January 2024, they released the digital single "Fuyu to Haru" (冬と春), and in July, they released "Atarashii新しい恋人達に), the theme song for the Fuji TV Monday 9 pm drama "Umi no Hajimari" (The Beginning of the Sea), as a digital single (which was later released on CD as their 22nd single in September).
Starting in August, they will hold their first arena co-headlining tour, "anti sleeps tour 2024," featuring 16 guest artists and consisting of 32 performances in 15 locations nationwide.

In April 2025, she released "ブルーアンバー," the theme song for the Kansai TV/Fuji TV Monday 10pm drama "Anataある未来より愛を込めて." In June, she released "Ai wo Komete Aru Mirai yori," the theme song for the new Mos Burger brand commercial "Taberu HAPPY." Furthermore, she wrote the theme song "幕が上がる" for the theatrical film "TOKYO MER ~Hashiru Kyūkyūmeishitsu~ Nankai Mission," which will be released on August 1st.
In December, they released "どうしてもどうしても" the theme song for NHK's winter sports program, as a digital single.

Starting in May 2026, back number will hold their first-ever five-stadium tour, "Grateful Yesterdays Tour 2026," the largest in their history, with nine performances in five locations across Japan. In addition, they have announced their first-ever Asian tour, with performances in Taipei, Seoul, and Hong Kong.