PURPOSE: To simplify its constitution and also generate desired torque when an object is put in motion in the desired direction by engaging a roller in a variable space formed between a rotary member and a fixed member.

CONSTITUTION: A rotary member 1 rotates around a shaft 2, and has the circumferential surface 3, and a plane shape fixed member 4 is arranged so as to be opposed to the rotary member 1. A roller 8 is engaged in a space formed between the circumferential surface 3 and the fixed member 4, and a force is applied so that the roller 8 can be moved in the space narrowing direction. When the rotary member 1 rotates in the counterclockwise direction, the roller 8 also starts to rotate. Since the roller 8 tries to move in the space further narrowing direction, its frictional force becomes still larger. By adjusting friction coefficient between the roller 8, the rotary member 1 and the fixed member 4 properly, the roller 8 can give inverse torque to the rotation of the rotary member 1 while sliding and engaging on the fixed member 4.