PURPOSE: To provide a seal-assembled structure of an air booster which can greatly reduce the number of assembling processes for an oil seal and thus enhance productivity and which can also reduce the number of part items and that of machining processes.

CONSTITUTION: This air booster includes an air cylinder 1; a hydraulic cylinder 2 connected to one end of the air cylinder 1; and an air piston 3 and a hydraulic piston 4 which operate in conjunction with each other within the air cylinder 1 and the hydraulic cylinder 2. An oil seal portion 6 is provided at the connection between the air cylinder 1 and the hydraulic cylinder 2 and is held in place at one end of the air cylinder 1 by a stopper wall 8 extended in the direction of the axis of the air cylinder 1, and a piston rod 7 provided integral with the air piston 1 is passed through the oil seal portion 6 and held.