A position detecting device for a vehicle and a seat position detecting device which are compact in structure and low in production cost are proposed, in which the absolute position from the reference point can be always detected. The position detecting device for a vehicle for detecting a position of a mobile object for the vehicle which is moved by a driving force of a rotating driving source includes a deceleration device having a two-stage hypo-cycloid mechanism decelerating the rotation amount of a rotation shaft rotated plural revolutions to one revolution or less by the driving source upon the mobile object being moved and a rotation sensor detecting a rotation output from the deceleration device as an absolute angle, wherein a position of the mobile object is detected based on the absolute angle detected by the rotation sensor.