PURPOSE: To provide a coupling capable of setting a free condition not to transmit torque between an input shaft and an output shaft at an optional time, in a hydraulic power transmission coupling to control torque transmission between relatively rotatable input shaft and output shaft according to rotating speed difference between the input and output shafts.

CONSTITUTION: A hydraulic pump to operate according to rotating speed difference is provided between an input shaft and an output shaft relatively rotatable. A high pressure chamber 46 to collect liquid pressure from the hydraulic pump is provided. Further, a closing means to close an orifice 54 connected to the high pressure chamber 46 when the liquid pressure in the high pressure chamber 46 is over a prescribed value, and a relief means to lose the pressure in the high pressure chamber 46 by an external signal are provided.