PURPOSE: To always provide the constant speed by constituting a mechanism integrally with a case, housing a central part of a drive shaft and some crescent- shaped members which collaborates with the central part in the case, and providing the crescent-shaped members inside a rotational race.

CONSTITUTION: A drive shaft (an input shaft) 1 is provided on a bush 7, a radially extending fins are provided on the busy 7 and inserted in a drag casing 9 provided on crescent-shaped members 2. The members 2 may have the same number as the fins of the shaft 1. These members 2 are rotated by being drawn by the fins of the drive shaft 1. The members 2 are concentrically rotated on a rotational race 3 and brought into contact with inserted brake rollers 6 in the radial direction. When driving force is supplied to an output shaft 5, the output shaft 5 having two flat side surfaces presses the rollers 6 against the members 2, braking force is applied to the brace 3, and the shaft 1 is not moved. Therefore, complete synchronism can be obtained between the speeds of the drive shaft 1 and the output shaft.