A first pump of small capacity and a second pump of large capacity are connected directly with each other and are driven by a variable-speed motor. The rotational speed of the variable-speed motor is controlled by a control device. In a first mode, a first discharge line of the first pump and a second discharge line of the second pump are disconnected with each other, making the first pump unloaded. With the first pump in the unloaded state, a constant-horsepower operation is performed, where the discharge fluid is brought into a high pressure by relatively small torque. In a second mode, the first discharge line and the second discharge line are connected with each other by a switching valve. With the first and second discharge lines in the connected state, a constant-horsepower operation is performed, where a high flow rate of discharge fluid is discharged at relatively low rotational speed.