An electric opening/closing device for vehicle, which has good layout characteristic of the vehicle and is provided with a highly-rigid attachment part, is provided. The vehicle electric opening/closing device comprises: a worm wheel meshed with a worm provided on a motor shaft of an electric motor; a planetary gear mechanism disposed coaxially with the worm wheel and decelerating rotation of theworm wheel to transmits it to an output gear; a gear case housing the worm wheel etc. and assembled to the electric motor; and a swing arm rotatably supported by the gear case between an output end linked to a back door and an input end linked to an output gear. An attachment plate for mounting the vehicle electric opening/closing device on the vehicle is sandwiched between the first case body and the second case body which constitute the gear case.