An object of the present invention is to provide an electric booster enabling an easy connection of an ECU and a stator. An ECU casing containing boards and (ECU) is attached to the outer surface of the casing body so as to be located near a stator of an electric motor. The opening of the casing body is closed by a rear cover. A flat-portion through-hole (through-hole) is formed through the casing body. Bus bars connecting the stator and the ECU are disposed through the flat-portion through-hole. The bus bars are disposed such that one ends thereof are coupled with the stator while the other ends thereof protrude from the flat-portion through-hole. When the electric booster is assembled, since the stator and the bus bars are disposed at the casing body having the flat-portion through-hole, it is possible to easily assemble the casing body and the rear cover with the bus bars inserted through the flat-portion through-hole.