Abstract: A brake system includes a pneumatic circuit provided with a first and a second passage that supply compressed air from an air tank that stores compressed air to a brake booster; and a control unit that controls a proportional control valve arranged on the second passage. The control unit includes a first signal generator configured to generate a first signal that is a pulse signal having a first frequency and indicates a drive amount of the proportional control valve. The control unit also includes a second signal generator configured to generate a second signal that is a pulse signal having a second frequency, which is less than or equal to one-tenth of the first frequency. The control unit further includes a third signal generator configured to generate a command signal for the proportional control valve by multiplying the first signal by the second signal.