The purpose of this study was to investigate the changes on mechanical work of the lower limb joints during baseball pitching in a simulated game. One male college baseball pitcher threw 15 pitches in an inning for 9 innings (135 pitches) in an indoor pitcher's mound with two force platforms. Rest time between innings was 6 minutes. Three-dimensional positions of 47 reflective markers attached to subject were tracked by an optical motion capture system (Vicon Motion System 612, Vicon Motion Systems) with eight cameras (250Hz). For subject 75 fastball pitches (1st, 3rd, 5th, 7th, and 9th innings) were chosen for analysis.As the main results, the hip joint extension absolute and negative work of the stride leg decreased with increasing the number of pitches. The ankle joint extension absolute and negative work of the stride leg increased with increasing the number of pitches. These results suggest that the hip joint extension torque of the stride leg was needed to maintain for higher performance in baseball pitching.