To compactly constitute a power transmission device with a simple structure, caused for performing torque transmission by the pump action to be caused with the relative rotation and provided with a mechanism for restricting the torque transmission.

A hub 12 and a housing 14 are relatively rotated, a pair of gear-type pumps 16, 18 generate fluid pressure and perform torque transmission. When fluid pressure is increased, pistons 38, 40 for constituting side walls of the pumps 16, 18 are retreated against energizing force of a compression coil spring 42, the flowing resistance of fluid to flow from the discharging side to the suction side is decreased, and rising of transmitting torque is restricted.