Back Number (stylized as back number) is a Japanese rock power trio formed in 2004. The group consists of Iyori Shimizu (lead vocals and guitar), Kazuya Kojima (bass guitar and backing vocals), and Hisashi Kurihara (drums). The group signed to independent record label Idolsmith Recordings in 2009 and released their debut EP Nogashita Sakana (逃した魚, Fish Set Free) (2009) and studio album Ato no Matsuri (あとのまつり, Too Late) (2010). They signed to Universal Sigma in 2011.
The group's biggest hit is "Christmas Song", which peaked at number-one for three weeks on the Japan Hot 100 chart, and was later certified million by the Recording Industry Association of Japan. Their first compilation album, Encore (2016), reached number one on the Billboard Japan Hot Album for two consecutive weeks in 2016 and also ranked number five on the Oricon yearly best-selling album chart of 2017. In 2018, back number successfully held a Dome Tour titled “stay with you”, including a 2-night-concert in Tokyo Dome. All tickets for 5 concerts are sold out within 2 hours, mobilized over 45,000 guests per concert.