The present invention relates to an electric booster in which a rotation of an electric motor is converted into a linear motion and transmitted to an output member by a rotation-linear motion mechanism, and the output member pushes a piston of a master cylinder to generate a brake fluid pressure in a pressure chamber in the master cylinder. The present invention is characterized in that an ECU casing containing boards and the like such as a board for controlling the electric motor 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 F side, and a space between the male connector and an interference area of the ECU casing with the other components and devices is utilized as a space for attachment/detachment of the male connector to/from the female connector.