Daisuke Matsuzaka
In 2003, Mitsui was 11–5 with a 5.22 ERA. He was second on Seibu in wins, behind Daisuke Matsuzaka. In 2004, Mitsui was limited to 9 games, with a 7.69 ERA and 67 hits in innings pitched. He was 2–3. He allowed one run in two innings in the 2004 Japan Series.