He previously played in NPB for the Yomiuri Giants and Hokkaido Nippon-Ham Fighters.

On September 25, 2013, he made his debut as a relief pitcher in the Central League against the Tokyo Yakult Swallows.

On November 2, 2016, Kumon and Taishi Ohta was traded to the Hokkaido Nippon-Ham Fighters in exchange for Mitsuo Yoshikawa and Shingo Ishikawa.

On August 29, 2019, Kumon broke the NPB record for undefeated pitchers in 165 games.

On August 12, 2021, Kumon and Shota Hiranuma was traded to the Saitama Seibu Lions in exchange for Fumikazu Kimura and Ryusei Satoh.