A position detecting device for detecting a position of a movable body which is moved by a drive system having a rotation body includes a magnetic part having an N-pole and an S-pole on a face of the rotation body and a sensor chip having at least a magneto-resistive element. The face of the rotation body is perpendicular to the rotation shaft of the rotation body and the sensor chip is provided so as to face the magnetic part for detecting a variation of magnetic field by the magneto-resistive element when the magnetic part is rotated with the rotation body. A little space is required for mounting the sensor chip having the magneto-resistive element and therefore downsizing can be attained.