Provided is a hydraulic power transmission device enabling to minimize a pressure loss in an oil passage downstream from a piston chamber of a hydraulic clutch, and to properly prevent reduction in transmission torque capacity in the hydraulic clutch. Left and right pressure regulating valves (8L, 8R) regulating pressure of oil flowing out from piston chambers (59L, 59R) of left and right clutch devices (5L, 5R) are arranged adjacent to a downstream of the piston chamber (59). This can shorten an oil passage from the piston chamber (59) to the pressure regulating valve (8) and minimize pressure loss in the oil passage. Due to better flexibility of arrangement of the pressing load receiving member (82) receiving a pressing force (thrust) of the piston member (57), thrust efficiency of the hydraulic piston (57) improves to properly prevent reduction in transmission torque capacity in the hydraulic clutch (5).