The invention improves accuracy of engagement of a pinion and a rack with each other in an automatic opening/closing device for a vehicle. In the automatic opening/closing device for the vehicle and for automatically opening and closing an opening/closing member provided to a vehicle body, the device includes a motor for generating a driving force for driving a pinion for rotation, a sliding member having a rack engaged with the pinion, and a rail member for positioning the sliding member and for guiding the sliding member in a predetermined direction. The sliding member and the opening/closing member are connected through a connecting rod. A base member is integrally formed with a motor fixing portion and a rail fixing portion. The motor and the rail member are fixed to the base member. As a result, it is easy to improve accuracy of positioning of the pinion and the rack and to improve accuracy of engagement of the pinion and the rack with each other.