On October 24, 2013, Kajiya was drafted by the Fukuoka Softbank Hawks in the first overall pick in the 2013 Nippon Professional Baseball draft.[1] In the 2014 -2015 season, he played in the Western League of NPB's minor leagues and played in informal matches against Shikoku Island League Plus's teams.
On August 26, 2016, Kajiya pitched his debut game as a relief pitcher against the Chiba Lotte Marines.[2] In the 2016 -2017 season, he pitched in four games. On July 14, 2018, Kajiya pitched in the 2018 NPB All-Star game [jp].[3] In the 2018 season, he pitched in 72 games as a setup man, matching the Hawks' record.[4] And he finished the regular season with a 4–3 win–loss record, a 3.38 ERA, 31 holds, 53 strikeouts in 66 2/3 innings, and he pitched in the 2018 Japan Series. In the 2019 season, Kajiya finished the regular season with 30 games pitched, a 3–1 win–loss record, a 6.00 ERA, and 23 strikeouts in 36 innings. On December 2, 2020, he became a free agent.[5]
