[1] He was 5th in the Pacific League in ERA behind Masahiko Kaneda, Koo Dae-sung, Nate Minchey and Kenichi Wakatabe.

In 2004, Mitsui was limited to 9 games, with a 7.69 ERA and 67 hits in 45+2⁄3 innings pitched.

Mitsui struggled in 2008 as the veteran faded to 1–1, 7.50 and allowed 33 hits in 24 innings.

[1] He allowed one run in 1+1⁄3 innings in the 2008 Japan Series, which Seibu won.

Mitsui requested to be posted by the Lions to Major League Baseball twice in the 2008–09 offseason, but attracted no bids on either occasion.