In a piston pump/motor of swash plate type which comprises: a cylinder block disposed in a casing to be rotatable jointly with a shaft therein; a plurality of pistons slidably inserted in the cylinder block parallel to the shaft; and a swash plate swingably attached to the casing for guiding the plurality of pistons on a front surface thereof and changing a displacement of each of the pistons by changing an inclination thereof, there is provided a swash plate angle changing apparatus in which the swash plate has a rear surface formed with a first contact surface and a second contact surface which make an angle between them. The casing has a swash plate support surface which is perpendicular to the shaft and with which the first contact surface and the second contact surface are each adapted to contact. The casing has a swash plate angle changing piston; the swash plate is swung by a force resultant from thrust forces of the pistons in the direction of the swash plate to a first position whereby the first contact surface makes contact with the swash plate support surface to yield a large angle of the said swash plate and also is swung by a thrust force of the swash plate angle changing piston to a position whereby the second contact surface makes contact with the swash plate support surface to yield a small angle of the swash plate; and the said swash plate has a center of swing located closer to the front surface than the rear surface of the swash plate.