A fuel system includes: a constant-volume pump and a centrifugal pump increasing the pressure of fuel to be supplied to an engine for aviation and 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 it using the centrifugal pump; and a speed changer connecting the engine and the centrifugal pump, changing the rotational speed of rotational power output from the engine and transmitting the rotational power to the centrifugal pump, and being capable of adjusting the speed-changing ratio of the rotational speed.