Patent ID: 8011184

Claim:
An electric booster, which drives an electric motor in response to a movement of an input member moving together with a brake pedal, assists a piston of a master cylinder through a rotation-linear motion converting mechanism, and generates a brake hydraulic pressure in the master cylinder, wherein: the electric motor comprises a stator including a coil, a hollow rotor in which the rotation-linear motion converting mechanism is fitted, and which rotates by applying a current to the stator, and an annular motor casing containing the stator and the rotor; an opening on one axial end side of the motor casing is formed on a front side thereof to which the master cylinder is connected, the opening having a larger diameter than an outer diameter of the rotation-linear motion converting mechanism; the rotation-linear motion converting mechanism comprises a nut member fitted to the hollow rotor so as to be non-rotatable relative to the hollow rotor, and a hollow screw shaft engaged with the nut member and disposed so as to be non-rotatable relative to the motor casing while being axially movable; and the piston of the master cylinder is inserted through the opening on the one axial end side of the motor casing so as to be disposed in the hollow screw shaft of the rotation-linear motion converting mechanism, resulting in abutment of the piston against the screw shaft inside the hollow screw shaft, the abutment functioning to limit movement of the piston in a backward direction opposite from the master cylinder.