In 2006, Yoshii marked a win against the Tohoku Rakuten Golden Eagles, becoming the fifth player in to have recorded victories against 12 Japanese professional baseball teams.

On April 25, 2007, the 42-year-old Yoshii started the game against the Rakuten Eagles with 18-year-old Masahiro Tanaka as the opposing pitcher and recorded a win giving up no runs over 5 innings of pitching.

This made him the sixth Japanese pitcher to have recorded a win at or above age 42, after Shinji Hamasaki, Tadashi Wakabayashi, Yoshinori Sato, Yutaka Ohno, and Kimiyasu Kudoh.

He continued to pitch during the season as a starter but was demoted to relief duty by manager Terry Collins, due to alcohol-induced rage.

He was released by the Marines on November 13, and announced his retirement to become a pitching coach for the Hokkaido Nippon-Ham Fighters under manager Masataka Nashida.