A swash plate type hydraulic pump or motor comprises first and second swash plates which move reciprocally while opposing first and second pistons, so as to expand and contract a volume chamber according to rotation of a cylinder block. Drive pistons push on the swash plates from behind, causing the first and second swash plates to tilt, respectively. A tilt angle control valve controls the tilt angles of the swash plates by selectively increasing drive pressures that are guided to the drive pistons. A port plate is provided in a sliding portion between the first swash plate and the first piston. The port plate rotates integrally with the cylinder block and guides high and low pressure side hydraulic fluid, which flows through the supply and discharge ports provided in a sliding surface of the first swash plate, to the volume chamber via an inner portion of each first piston.