A pump volume control apparatus configured to change a pump volume of a pump in accordance with a tilt angle of a swash plate, the pump volume control apparatus comprising: a tilting piston configured to tilt the swash plate in a direction to reduce the pump volume as a tilt driving pressure (Pc) becomes higher; a pump volume switching valve configured to adjust the tilt driving pressure (Pc) in response to a movement of a spool; a flow rate control spring configured to bias the spool in accordance with the tilt angle of the swash plate; and a horsepower control piston configured to move in accordance with a pump discharge pressure of the pump; characterized in that the pump volume control apparatus further comprises: a horsepower control spring (31, 32) configured to bias the horsepower control piston in accordance with the tilt angle of the swash plate; and a rod provided between the horsepower control piston and the spool, wherein (i) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the spool in response to a flow rate controlling signal pressure (Pi) in a flow rate controlled state where a gap is formed between the rod and the spool, and (ii) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the horsepower control piston in response to the pump discharge pressure in a horsepower controlled state where the rod is in contact with the spool. a tilting piston configured to tilt the swash plate in a direction to reduce the pump volume as a tilt driving pressure (Pc) becomes higher; a pump volume switching valve configured to adjust the tilt driving pressure (Pc) in response to a movement of a spool; a flow rate control spring configured to bias the spool in accordance with the tilt angle of the swash plate; and a horsepower control piston configured to move in accordance with a pump discharge pressure of the pump; characterized in that the pump volume control apparatus further comprises: a horsepower control spring (31, 32) configured to bias the horsepower control piston in accordance with the tilt angle of the swash plate; and a rod provided between the horsepower control piston and the spool, wherein (i) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the spool in response to a flow rate controlling signal pressure (Pi) in a flow rate controlled state where a gap is formed between the rod and the spool, and (ii) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the horsepower control piston in response to the pump discharge pressure in a horsepower controlled state where the rod is in contact with the spool. a horsepower control spring (31, 32) configured to bias the horsepower control piston in accordance with the tilt angle of the swash plate; and a rod provided between the horsepower control piston and the spool, wherein (i) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the spool in response to a flow rate controlling signal pressure (Pi) in a flow rate controlled state where a gap is formed between the rod and the spool, and (ii) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the horsepower control piston in response to the pump discharge pressure in a horsepower controlled state where the rod is in contact with the spool. (i) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the spool in response to a flow rate controlling signal pressure (Pi) in a flow rate controlled state where a gap is formed between the rod and the spool, and (ii) the tilt driving pressure (Pc) is adjusted by means of the movement of the spool in accordance with a force acting on the horsepower control piston in response to the pump discharge pressure in a horsepower controlled state where the rod is in contact with the spool.