An outer circumferential surface of a trunnion shaft includes a sliding contact region that has a circular arc shape as viewed along a slanting axis line and is substantially brought into contact with an inner circumferential surface of the bearing portion in a rotatably sliding manner, and at least one suction groove that forms a pocket portion between the suction groove and the inner surface of the bearing portion. The pocket portion is opened to an inner space of housing on at least one side in an axis line direction of the trunnion shaft.