PURPOSE: To improve the durability by transmitting power through a reaction plate contacting the power piston in a larger area.

CONSTITUTION: An thrust of a power piston 3 is transmitted to an output shaft from a inner periphery 13b of a reaction plate 13 through reaction disc 16. At this point, a backward reaction working on the output shaft 17 is transmitted to the power piston from the inner periphery 13, while deforming the reaction disc 16 under pressure causing a part thereof to be protruded into a hole 13a. As a result, the reaction is transmitted to a brake pedal via a plunger 9 and an output shaft 8. The ratio can be latered simply by changing either the diameter of the cylinder section 14a of a retainer 14 or the diameter of the hole 13a on the reaction plate 13.