Abstract: This invention relates to an axial piston system hydraulic pump or motor capable of being changed over by two stages and particularly preferably used as the hydraulic motor for driving vehicles such as bulldozer, shoveldozer, etc. Thus, to provide as compact as possible two step changing-over mechanism and a well-balanced operation with a relative small force, a smooth plane against which the shoe at the plunger side opposed to a swash plate abuts and a first and second abutting portions located at the opposite plane to the smooth plane to abut against a fixed portion, when the swash plate pivots about the center of pivotal movement, to regulate the angle of said smooth plane of the swash plate to two positions are formed while a drive member is disposed which pivots said swash plate about the center of the pivotal movement to displace the angle of the smooth plane to two positions.