Hatsukoi Cider / DEEP MIND (初恋サイダー/DEEP MIND; First Love Cider / DEEP MIND) is the 13th single released by Buono!. This is the first Double A-side released by Buono!. "DEEP MIND" is used as the theme song for Gomennasai, a movie which Buono! stars in. "Hatsukoi Cider" is the ending theme to the drama Suugaku♥Joshi Gakuen. There are two editions available: Regular and Limited. This is Buono!'s third single under the zetima label.
On October 29, 2011, "DEEP MIND" was released for download on iTunes. It was later released as a double A-side CD single with "Hatsukoi Cider" on January 18, 2012. The Single V and Event V were released on January 25, 2012, and February 26, 2012, respectively.
A Twitter post by user @w__cmt on November 28, 2019 featuring a clip of Buono! performing "Hatsukoi Cider" at Buono! Festa 2016 went viral on Twitter in Japan, garnering over 1 million views, 12,400 retweets, and 58,500 likes in just one day. This viral clip rejuvenated interest in Buono!, causing the group to trend at a peak position of #3 on Twitter in Japan the following day. In response, Suzuki Airi posted a Tweet of her own asking why everyone was talking about Buono! again, and expressing her happiness to the newfound attention.