To minimize a hydraulic pump by forming a recess opened at its one side and a first relief hole communicated with a tank on a pump housing, and mounting an engagement body having a second relief hole communicated with the first relief hole and opened and closed by a valve element, and the valve element in the recess.

When the gears at driving side and driven side are rotated by the driving of an electric motor, the hydraulic fluid in a tank 1 is sucked in a pump chamber, further the pressurized liquid is generated every time when each liquid chamber is opened to a discharge passage, and this pressurized liquid is supplied to a damper chamber 43 through the discharge passage. When the liquid pressure in the damper chamber 43 rises above a predetermined value, the valve element 51 supported in a recess 20 communicated with the damper chamber 43 through a communication hole 42 is opened against the energization force of a valve spring 52, whereby the recess 20 and the relief holes 30, 27 are communicated with one another, and the pressurized liquid of the damper chamber 43 is returned to the tank 1. On this occasion, as an engagement body 26 comprises an annular groove 31 communicated with the relief holes 30, 27 and the throttle part 32, the liquid pressure can be reduced in the annular groove 31.