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, 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 (2018), reached number one on the Billboard Japan Hot Album for two consecutive weeks in 2016 and was the world best-selling album of the second week of 2017.