Abstract: A ridge spirally wound around an outer peripheral cam is held between each pair of followers of a rotatable body including a plurality of such followers planted therearound at equal intervals. The ridge of the outer peripheral cam consists of a gentle slope portion having a minute but substantial lead angle other than zero degree and steep slope portions having large lead angles and being continuous from the gentle slope portion at opposite ends thereof. The steep slope portions at opposite ends of the ridge are juxtaposed to each other in parallel relationship. The rotatable body is rotated at a high speed when the followers are engaged with the steep slope portions but rotated at a slow speed when the followers are engaged with the gentle slope portion of the ridge, as the outer peripheral cam is rotated. The output shaft is connected to the rotatable body. The outer peripheral cam is driven by a control motor adapted to be stopped at a desired angular position.