The purpose of this study was to investigate the mechanism of acceleration of the hammer head in the hammer throw by analyzing the motions of the hammer (hammer head, wire, and handle). The study subjects were 10 male throwers (PB: 43.15-68.21 m). Hammer motions were videotaped with high-speed VTR cameras (250 or 200 fps), and the three-dimensional coordinates were calculated by a DLT method. Kinematic parameters such as hammer head speed, hammer head acceleration, increase in hammer head speed in each turn phase, and displacement of the leading handle (hand) were calculated. In addition, a computer simulation was performed to verify the fundamental factors involved in hammer head acceleration. These factors were defined as follows. (1) The handle (hand) with the positive leading distance, which was defined as the distance from the handle to a line connecting the hammer head and the instantaneous center of rotation of the hammer head, had a positive effect on increase in the hammer head speed. In contrast, the handle with the negative leading distance had an effect on decrease in the hammer head speed. (2) The normal acceleration of the hammer head, which resulted from the pulling force of the hammer head toward the instantaneous center of rotation, had no direct effect on the increase in the hammer head speed. (3) The effect of gravity on the increase in the hammer head speed increased gradually with the number of turns. In other words, we reconfirmed that the handle position relative to the instantaneous center of rotation was the dominant determinant of the increase and decrease in the hammer head speed. In addition, from a biomechanical viewpoint, we confirmed that a decrease in the radius of curvature of the hammer head had no direct effect on the increase in hammer head speed.