The first round of the men's golf domestic tour tournament was held in Hokkaido, and South Korea's Song Young-han took the lead with a 7-under.
Son, who has won one tour, went around with one eagle, six birdies and one bogey, marking a 7-under mark to start the lead.
Toshiya Takeyasu, Tomoyasu Sugiyama, Kunihiro Uei, and Scott Strange of Australia are in second place with a one-stroke difference.
Ryo Ishikawa, who won the previous tournament in 2019, is ranked 27th with 3 unders along with Shugo Imahira and Takumi Kanaya.
Rikuya Hoshino, who participated in the Tokyo Olympics, was behind in 68th place with 1 under.