PURPOSE: To prevent the generation of hammering noise between a cam and a plunger by using oil pressure in place of a return spring for pressing the plunger to the cam side in a hydraulic power transmission joint.

CONSTITUTION: Oil pressure is supplied from a communicating groove 12 communicated with a cylinder hole 8 so that a return piston 11 is reciprocated in the cylinder hole 8 integrally with a plunger 5. At the time of supplying oil pressure, oil discharged from a plunger chamber 6 is collected into a high pressure chamber 18, and this oil pressure is reduced by a pressure reducing means 43. In this case, an orifice 23 is closed by a first relief means 28 at the specified pre-load to secure the lowest oil pressure. At the release time, a second relief means 33 is opened to eliminate the pressure of the communicating groove 12. Consequently, there is no load to push the plunger 5 back to the cam 2 side, so that the plunger 5 after a discharge stroke is put in the withdrawn state. Hammering noise caused by collisions between the cam and plunger is not therefore generated.