A sensor of a position detecting device includes a sensor body, a magnetic detector, and at least one engagement protrusion that extends from a position in the vicinity of the magnetic detector. The engagement protrusion engages with an engagement hole disposed on a casing. An anti-rotation hole is disposed on the casing in a position spaced apart from the engagement hole. An anti-rotation pin is disposed on the sensor body, and engages with the anti-rotation hole to prevent the casing and the sensor body from rotating relatively to each other about the engagement hole. At least a part of the engagement protrusion is in contact with an inner wall of the engagement hole on a straight line L connecting a center of the magnetic detector and a center of the anti-rotation pin.