Abstract: A braking force holding device for an air brake system, maintains a braking force when a vehicle is stopped facing upwardly on a hill to facilitate the start of the vehicle on the hill. The braking force holding device includes a main body having an inlet port connected to a brake valve, an outlet port connected to a pneumatic relay valve, a control port connected to the pneumatic actuator of a clutch operating device, and a passage interconnecting the inlet port and the outlet port; a valve device capable of intercepting the flow of air from the outlet port toward the inlet port; and a control unit for controlling the valve device.