An electric power steering device (1) according to the present invention includes: a gearbox; and a driving control unit (4) connected to the gearbox through a mounting-portion contact surface (7). The driving control unit (4) includes an outer-diameter gradually increasing surface (11) formed on one end portion thereof, the outer-diameter gradually increasing surface (11) being connected so that an outer diameter thereof on the mounting-portion contact surface (7) matches with an outer diameter of the gearbox. Thus, it is possible to prevent a liquid from entering inside through the mounting-portion contact surface (7), thereby preventing electrical insulating properties of internal electric components from being lowered.