There is provided a mobile member position detection apparatus in which not only high-accuracy detection of a window glass or the like of a power window apparatus but also setting of an initial position being free from cumbersome adjustment can be realized with a simple structure at low cost. In a position detection apparatus, a ring gear is rotated together with a motor output shaft, and a predetermined rotational position is detected by a mobile contact and a fixed contact. In this case, since a projecting portion of a gear base is brought into contact with a contact surface of a base plate to hold the gear base, a difference between loss torque of the gear base and loss torque of the ring gear increases, and a planetary gear is not unnecessarily revolved. Thereafter, when the output shaft is rotated, holding of the carrier by a wave washer is released, and rotation of the output shaft is not transmitted to the ring gear. Therefore, when the output shaft is sufficiently rotated in an incorporating operation, an initial state can be automatically set.