In September 2010, Mizushima left his talent agency, Ken-On, to create an own independent agency called A stAtion. Several weeks later, on 31 October 2010, it was announced that Mizushima had won the 5th Poplar Publishing Grand Prize for Fiction and a cash prize of 20 million yen for his debut work, titled Kagerou, written under the pen name Satoshi Saitō (齋藤 智, Saitō Satoshi) but he decided to decline the 20 million yen prize money. The book was published in December 2010. In the two and a half years of Oricon's book charts, Kagerou recorded the third highest first-week sales, behind J. K. Rowling's Harry Potter and the Deathly Hallows (1.191 million) and Haruki Murakami's 1Q84 Book 3 (398,000).