An electric motor driving device comprising an electric motor unit that includes an electric motor body and a motor case accommodating the electric motor body, and a planetary gear unit that includes a planetary gear mechanism decelerating rotative power input from the electric motor body, a planetary output shaft outputting the rotative power decelerated by the planetary gear mechanism and a planetary case accommodating the planetary gear mechanism and connected to the motor case, wherein the planetary gear mechanism has a sun gear that inputs rotative power from the electric motor body via a power transmission shaft, a planetary gear that meshes with the sun gear, a carrier pin that supports the planetary gear in a relatively rotatable manner with respect thereto and that orbits around the sun gear together with the planetary gear, an internal gear whose inner circumferential edge meshes with the planetary gear and that functions as a fixed element, and a carrier member that supports the carrier pin and to which the planetary output shaft is connected, the electric motor driving device being characterized in that, a motor-side first side wall of the motor case that is brought into contact with the planetary case is formed with an output opening into which the power transmission shaft is inserted, the carrier member includes a carrier-side first side wall that is located on a side closer to the electric motor unit than the planetary gear, that is formed with a central opening into which the power transmission shaft is inserted, and that supports the carrier pin, a carrier-side second side wall that is located on a side farther from the electric motor unit than the planetary gear and that supports the carrier pin, and connecting pieces connecting the carrier-side first and second side walls (221, 222) while permitting the planetary gear to partially extend more radially outward than the carrier-side first and second side walls (221, 222), the planetary case has a planetary-side first side wall close to the motor case with respect to the axial direction, a planetary-side second side wall located on the side opposite to the motor case with respect to the axial direction, and a planetary-side peripheral wall connecting the outer peripheral edges of the planetary-side first and second side walls (255, 260), the planetary-side first side wall is formed, in the center in the radial direction, with an input opening into which the power transmission shaft is inserted, and has an inner region extending radially outward from an outer peripheral edge of the input opening, an axially extending region extending in the direction toward the carrier-side first side wall from an inner end in the radial direction of the inner region, and an outer region extending radially outward from the inner region via a transitional region, the carrier-side first side wall is rotatably supported via a bearing member inserted between an inner peripheral surface of the central opening and an outer peripheral surface of the axially extending region, and the inner region protrudes in a direction toward the electric motor body in comparison with the outer region in an axial direction of the power transmission shaft so that at least a part of the inner region enters the motor case via the output opening of the motor-side first side wall when the motor case and the planetary case are connected.