A camera module includes a lens holder (10), a lens module (20), a position detecting mechanism (30), and an image pick-up module (50). The lens holder has a recessed portion axially defined in an inner periphery thereof adjacent one end thereof. The lens module is axially movably received in the lens holder. The position detecting mechanism includes a light source (32) disposed in the periphery of the lens holder opposite to the recessed portion to emit a light, and a photo-detector (34) securely received in the recessed portion, and a processor (36). The photo-detector has a plurality of photo-detector components (342) arranged in the recessed portion parallel to each other so that each can separately receive the lights from the light source and transform the light into an electrical signal. The processor is electrically connected with each photo-detector component for transforming the electrical signal into an output signal.