Koga was born in Niigata Prefecture on October 28, 1998. He joined J1 League club Kashiwa Reysol from their youth team in 2017. In June 2018, he moved to J2 League club Avispa Fukuoka on loan. He played many matches as center back. In 2019, he returned to Kashiwa which was relegated to J2 from 2019. Kashiwa was returned to J1 in a year.