The 2004 Nippon Professional Baseball season was the 55th season of operation for Nippon Professional Baseball (NPB). For the first time since 1982, the Pacific League instituted a playoff system to determine its representative to the Japan Series, in which the second-place and third-place teams would compete against each other for the right to play the regular season champion; previously, the league had a split-season playoff that would match the first-place team of the first and second half of the season from 1973 to 1982. The Pacific League regular season champion Fukuoka Daiei Hawks became part of dubious history when they subsequently lost to the Seibu Lions in five games to become the first regular season champion of the league to not reach the championship series. The Japan Series ended with the Seibu Lions defeating the Chunichi Dragons in the 2004 Japan Series. This season also saw the first and only players strike in Japanese professional baseball history. Players went on strike for two days in September because of the potential mergers and realignment. The result was that the Pacific League saw the Tohoku Rakuten Golden Eagles be created as an expansion team for 2005 while the Osaka Kintetsu Buffaloes and Orix BlueWave would merge to form the Orix Buffaloes.
Note:Two games for each team are cancelled due to players' strike
Seibu Lions (2) vs. Hokkaido Nippon-Ham Fighters (1)
Fukuoka Daiei Hawks (2) vs. Seibu Lions (3)
Chunichi Dragons (3) vs. Seibu Lions (4)