There is provided a position detecting device using reflection type photosensors in which a position sensing of lens located not less than 1 mm apart from a sensor can be conducted.A pair of reflection type photosensors PR1 and PR2 are oppositely arranged, a double sided reflector 5 attached on a movable body is movably arranged between the pair of reflection type photosensors and a position of the double sided reflector 5 is detected from the outputs of these reflection type photosensors. In the position detecting device of the present invention, an operating formula in which linear values are obtained depending on a moving distance of the double sided reflector can be used. For example, when an output of one of the pair of reflection type photosensors is Vo1, and an output of the other is Vo2, the position detecting is conducted using the operating formula of (Vo1−Vo2)/(Vo1+Vo2).