Abstract: A pneumatic-hydraulic booster for vehicle includes a master cylinder part; a pneumatic cylinder part combined with the master cylinder part which includes a power piston slidable fitted to a cylindrical casing and an output rod fixed to the power piston, extending to the master cylinder part; a partition wall body fitted to the cylindrical casing; and a relay valve part arranged at the opposite side of the partition wall body from the power piston in the cylindrical casing. The relay valve supplies compressed air into an air pressure chamber formed between the partition wall body and the power piston and discharges the compressed air from the air pressure chamber to the atmosphere. When compressed air is suppplied into the air pressure chamber through the relay valve part the power piston and the output rod are moved forwards to generate hydraulic pressure in the master cylinder. The hydraulic pressure is applied to a wheel cylinder for braking.