An opening/closing system for vehicle can improve detection accuracy of rotation of an output member and enhance reliability of its operation. The opening/closing system for vehicle includes a drive unit having an electric motor and a reduction gear unit. A drum, which is an output member, is fixed to the output shaft of the drive unit through a power transmittable member. A magnet fixed to the outer periphery of a disk member is disposed between the drum and the gear case of the reduction gear unit in such a way that the magnet rotates along with the drum when the disk member is fixed to the power transmittable member. A housing case fixed to the gear case is provided with a sensor accommodating section that accommodates a magnetic sensor. The magnetic sensor is disposed to detect the rotation of the drum from a change in a magnetic field created by the magnet.