A fuel system for an engine for aviation, the fuel system comprising: a constant-volume pump and a centrifugal pump configured for increasing the pressure of fuel to be supplied to the engine and configured for discharging the fuel; an operation controller configured to select in accordance with the operation state of the engine, one of a constant-volume pump-using mode of increasing the pressure of fuel using the constant-volume pump and a centrifugal pump-using mode of increasing the pressure of fuel using the centrifugal pump; and a speed changer for connecting the engine and the centrifugal pump, for changing the rotational speed of rotational power output from the engine, and for transmitting the rotational power to the centrifugal pump, and being capable of adjusting the speed-changing ratio of the rotational speed; characterized in that the fuel system further comprises: a first flow passageway in which the centrifugal pump is provided; a second flow passageway in which the constant-volume pump is provided; a shut-off mechanism provided on an upstream side of the centrifugal pump in the first flow passageway, and configured to block the first flow passageway when the fuel system is in the constant-volume pump-using mode and to open the first flow passageway when the fuel system is in the centrifugal pump-using mode; a third flow passageway whose first end is connected to a downstream end of the first flow passageway and to a downstream end of the second flow passageway, and whose second end is connected to an upstream side of the constant-volume pump in the second flow passageway; and a switching mechanism configured to connect the downstream end of the second flow passageway and the third flow passageway when the fuel system is in the constant-volume pump-using mode, and to connect the downstream end of the first flow passageway and the third flow passageway when the fuel system is in the centrifugal pump-using mode. a constant-volume pump and a centrifugal pump configured for increasing the pressure of fuel to be supplied to the engine and configured for discharging the fuel; an operation controller configured to select in accordance with the operation state of the engine, one of a constant-volume pump-using mode of increasing the pressure of fuel using the constant-volume pump and a centrifugal pump-using mode of increasing the pressure of fuel using the centrifugal pump; and a speed changer for connecting the engine and the centrifugal pump, for changing the rotational speed of rotational power output from the engine, and for transmitting the rotational power to the centrifugal pump, and being capable of adjusting the speed-changing ratio of the rotational speed; characterized in that the fuel system further comprises: a first flow passageway in which the centrifugal pump is provided; a second flow passageway in which the constant-volume pump is provided; a shut-off mechanism provided on an upstream side of the centrifugal pump in the first flow passageway, and configured to block the first flow passageway when the fuel system is in the constant-volume pump-using mode and to open the first flow passageway when the fuel system is in the centrifugal pump-using mode; a third flow passageway whose first end is connected to a downstream end of the first flow passageway and to a downstream end of the second flow passageway, and whose second end is connected to an upstream side of the constant-volume pump in the second flow passageway; and a switching mechanism configured to connect the downstream end of the second flow passageway and the third flow passageway when the fuel system is in the constant-volume pump-using mode, and to connect the downstream end of the first flow passageway and the third flow passageway when the fuel system is in the centrifugal pump-using mode. a first flow passageway in which the centrifugal pump is provided; a second flow passageway in which the constant-volume pump is provided; a shut-off mechanism provided on an upstream side of the centrifugal pump in the first flow passageway, and configured to block the first flow passageway when the fuel system is in the constant-volume pump-using mode and to open the first flow passageway when the fuel system is in the centrifugal pump-using mode; a third flow passageway whose first end is connected to a downstream end of the first flow passageway and to a downstream end of the second flow passageway, and whose second end is connected to an upstream side of the constant-volume pump in the second flow passageway; and a switching mechanism configured to connect the downstream end of the second flow passageway and the third flow passageway when the fuel system is in the constant-volume pump-using mode, and to connect the downstream end of the first flow passageway and the third flow passageway when the fuel system is in the centrifugal pump-using mode.