"Eien" is Zard's 22nd single released on August 20, 1997, under B-Gram Records label. Junichi Watanabe, the writer of the novel, wrote the title word "Eien" of the CD cover. The single reached rank number 1 first week. It charted for 13 weeks and sold 628,000 copies. When Izumi Sakai died in 2007, it was selected as her fifth best song on the Oricon polls.