PURPOSE: To reduce the relative net striking speed to cam faces in a striking and to reduce the impact sound by giving an angular velocity autorotating in the direction same as the reversal direction of the striking time to impact elements prior to the striking.

CONSTITUTION: During the revolution of an input shaft 23 and the revolution of weights 30 and 31 around the input shaft 23 together with the input shaft 23, an inertial energy is accumulated, and impact elements 33 relating to the weights 30 and 31 strike to the cam faces 102 of an output shaft, and the inertial energy is transmitted to the output shaft. In this case, when the impact elements 33 slide on a leading-in slope S of the output shaft in their revolving operations, the angular velocity autorotating in the direction same as the reversal direction in the striking is given to the impact elements 33. As a result, the relative net striking speed between the impact elements 33 and the cam faces 102 is reduced as the amount of the angular velocity given by the repulsion angular velocity generated by converting from the revolution to the autorotation of the weights 30 and 31 by the impact elements 33, so as to reduce the impact sound.