To provide an electric booster which does not give a sense of incongruity to a driver even during regenerative cooperation.

The electric booster 10 uses an input of a push rod 18 linked with a brake pedal 15 and the thrust of a booster piston 19 moved forward and backward by an electric actuator 12 using an electric motor 33 as a drive source for driving a piston 2 of a master cylinder 1 to generate brake hydraulic pressure in a pressure chamber 4 of the master cylinder 1. A simulator 16 is provided for generating a reaction force for withstanding the pedal force of the brake pedal 15. During operating the brake pedal 15, the booster piston 19 is controlled to be moved more than the push rod 18 to form a clearance between the push rod 18 and the piston 2. During regenerative cooperation to move the booster piston 19 backward, a returning force transmitted from the piston 2 to the push rod 18 via the clearance is canceled not to give a sense of incongruity to the driver.