An object of the present invention is to provide an automatic opening and closing apparatus for vehicle, which is improved so that the amount of lengthwise movement of an opening and closing element in the longitudinal direction of the vehicle body is increased as much as possible. An automatic opening and closing apparatus for vehicle, which moves a slide door 13 to open and close an opening section 11, the automatic opening and closing apparatus comprises: a guide rail 16 located along the longitudinal direction of the vehicle body 14; a cable 26 connected to the slide door 13; and a rear-side pulley unit 29 located on the vehicle body 14, wherein the guide rail 16 and the rear-side pulley unit 29 are located at positions different from each other in a width direction of the vehicle body 14, the rear end of the guide rail 16 and the rear-side pulley unit are located at the same position in the longitudinal direction of the vehicle body 14, and the automatic opening and closing apparatus further comprises: an auxiliary member which keeps the cable 26, which is routed between the slide door 13 and the rear pulley unit 29, in a predetermined state.