An electric booster which drives an electric motor (40) in response to a movement of an input member (9) moving together with a brake pedal (B), and generates a brake hydraulic pressure in a master cylinder (10) through a ball screw mechanism (50). The electric motor (40) includes a stator (41) having a coil therein, a rotor (42) which rotates by applying current to the stator (41), and an annular motor casing (2) containing the stator (41) and the rotor (42). The ball screw mechanism (50) is fittedly disposed in the rotor (42). An opening (3b) on one axial end side of the motor casing (2) has a larger diameter than an outer diameter of the ball screw mechanism (50). In manufacturing, after the electric motor (40) alone is tested, the ball screw mechanism (50) is inserted through the opening (3b) and installed without the need of disassembling the motor casing (2).