In an electric motor-driven booster, a sharp change of reaction force to an operation of a brake pedal is reduced to improve the brake pedal operation feeling. Operation of the brake pedal causes an input rod (34) and an input piston (32) to advance, and an electric motor (40) is driven according to the movement of the input piston to propel a primary piston (10) through a ball-screw mechanism, thereby generating a desired brake hydraulic pressure in a master cylinder. At this time, the input piston receives the hydraulic pressure in the master cylinder to feed back a part of reaction force during braking to the brake pedal. When the brake pedal is further depressed after the output of the electric motor has reached a maximum output and the primary piston 10 has stopped, a lock nut abuts against a movable spring retainer and compresses a reaction force spring.