ELECTRIC BOOSTER

PROBLEM TO BE SOLVED: To provide an electric booster which detects the operation of a brake pedal by a driver during the execution of automatic brake control.SOLUTION: An electric motor 27 is controlled on the basis of the operation of a brake pedal 24 to move a primary piston 12 forward through a ball screw mechanism 28, thereby generating a hydraulic pressure in a master cylinder 2. At this time, a reaction force from the hydraulic pressure in the master cylinder 2 is fed back to the brake pedal 24 through an input piston 21. Automatic brake control, which generates the hydraulic pressure by driving the electric motor regardless of whether there is the operation of the brake pedal 24, is carried out. During the automatic brake control, a determination threshold value Xs set on the basis of the relative position X between the primary piston 12 and the input piston 21 which is generated by a movement of the primary piston 12 is compared with an actual relative position X. The brake pedal 24 is determined to be operated, when the relative position X is smaller than the determination threshold value Xs.