Yamaguchi joined Yokohama in the first round of the high school draft in Heisei 18. After being traded to the Blue Jays of the big leagues and being out of the lineup, he returned to the Giants midway through the season.
Yamaguchi pitched in 17 games, posting a 443-66 record with a 66-save ERA of 112.3. He had hoped to continue playing, but on the 1th, he updated his Instagram and posted that he had decided to end his 17-year baseball career.