Japan scored in every inning and starting pitcher Kenta Maeda (1-0) allowed one hit and struck out nine over five innings in assuring two-time defending champion Japan a spot in the WBC semifinals.
The 12-run spread brought the WBC mercy rule into play and the game was called after seven innings.
Cuba and the Netherlands play an elimination game Monday to decide the other semifinalist from Pool 1. The winner of that game will play Japan Tuesday for seeding purposes. The semifinals begin March 17 in San Francisco.
Takashi Toritani hit the second pitch of the game for a home run and Nobuhiro Matsuda added a two-run shot and Seiichi Uchikawa a three-run homer in the second for a 6-0 lead. All those runs came off Netherlands starter Robbie Cordemans (0-1), who gave up six runs on five hits and two walks in 1 1/3 innings.
Atsunori Inaba had a third-inning home run while Yoshio Itoi's three-run homer in the fourth made the score 10-0.
Shinnosuke Abe had an RBI double in the fifth and Matsuda doubled in another in the sixth. Wladimir Balentien hit a three-run double and scored in the Netherlands' sixth that made the score 12-4 but Japan answered with Sakamoto's grand slam the next inning.