An automatic opening/closing apparatus for vehicle including a detected subject and a detection sensor for detecting rotation of an output shaft is downsized. A worm gear mechanism decelerating a rotation speed of an electric motor to output it from the output shaft is accommodated in a gear case, and a drum is fixed to a tip portion of the output shaft projecting from the gear case to the outside, whereby motive power of the output shaft is transmitted from this drum to a sliding door. A concave portion centering at an axial center and recessed in an axial direction is formed in a worm wheel constituting the worm gear mechanism, a magnet unit constituting a rotation sensor is disposed inside the concave portion of the worm wheel, and a magnetic sensor for detecting rotation of the magnet unit is fixed to an inner surface of the gear case.