Disclosed herein is an oscillating vane type pump actuator. The pump actuator of the present invention provides a method of preventing fretting corrosion from being caused on joined surfaces of elements. A cylinder (3c) has a comparatively low radial strength, and each of side covers (1c) and (2c) has a high radial strength. A cylindrical portion (1c-c), (2c-c) is provided on each side cover. Thus, when high pressure of work oil distorts the cylinder into a shape in which the cross-section of the cylinder becomes an ellipse-like shape, the cylindrical portions of the side covers act such that they are distorted in the same shape as that of the cylinder. Further, a passage that always communicates with a low-pressure side working chamber is formed in the contact surfaces between a fixed vane that is fixed to the cylinder and the side covers.