An electric booster in which a rotation of an electric motor is converted into linear motion and transmitted to an output member by a rotation-linear motion mechanism. The output member pushes a piston of a master cylinder to generate brake fluid pressure in a pressure chamber in the master cylinder. An ECU casing containing boards is coupled to the top of a motor casing constituting the electric motor. The ECU casing is offset in one direction in a plane perpendicular to a plane containing an axis of the master cylinder. A male connector is provided at a side surface of the ECU casing on the opposite side from the offset direction side, and a space between the male connector and an interference area of the ECU casing is utilized as a space for attachment/detachment of the male connector.
