PURPOSE: To get constant braking force by providing a piston part which converts input shaft rotational motion to linear reciprocating motion by means of a cam device, a cylinder part which contains the piston part and is filled with oil, an oil discharge part composed of an adjustable opening orifice and an oil inlet part having a check valve.

CONSTITUTION: When a follower part 15 comes into contact with a slant surface part 62, which extends to the top part 63 of a cam part 14, by rotation of the cam part 14 of an input rotary shaft 11, the follower part 15 descends together with a piston part 16 inside a cylinder 12 to the lowest position, and oil contained in the cylinder part 12 flows out through an orifice 19 toward the outside of the cylinder part 12. Accordingly, large frictional resistance is produced on a contact surface 51 between the follower part 15 and the cam part 14 because of action of oil resistance force, and torque of the input rotation shaft 11 is reduced. Four cylinder parts 12 are arranged, and since either one pair of two follower parts 15 are always pressed against the cam part 14, the torque is always reduced. In addition, it is possible to change degree of the torque reduction of the input rotary shaft 11 by changing rate of oil flow out of the orifice 19 by turning an orifice opening adjusting part 40 composed of a bush 72.