Miyaichi was born in Okazaki on December 14, 1992. After graduating from Chukyo University Chukyo High School, he signed with English Premier League club Arsenal in 2011. He loaned out to Dutch Eredivisie club Feyenoord soon. In summer 2011, he returned to Arsenal. He played 2 matches at FA Cup. In January 2012, he moved to Bolton Wanderers on loan. In the 2012/13 season, he loaned out to Wigan Athletic. However he could not play many matches for injury. In summer 2013, he returned to Arsenal and played for the club 1 season. However there were few opportunities to play. In the 2014/15 season, he loaned out to Eredivisie club Twente. In summer 2015, he moved to German 2. Bundesliga club St. Pauli. 