PURPOSE: To reduce the manufacturing cost of a moving plate where a condenser lens is formed in an optical shift detecting apparatus, to improve the reliability and to enhance the condensing efficiency.

CONSTITUTION: This optical shift detecting apparatus consists of a moving plate 1 shifting along a plane, a fixed light source 4 for casting incident light to the moving plate 1, and a fixed photodetecting element 5 placed to face the fixed light source 4. An image forming means, e.g. a band-shaped lens 3 extends on the moving plate 1, condenses the incident light in accordance with the shift of the moving plate 1 and forms a moving image. The fixed photodetecting element 5 has a photodetecting surface 6 which is arranged in the moving direction of the formed image, and outputs a detecting signal of the shift of the moving plate. The band-shaped lens 3 is formed of, e.g. a spiral Fresnel lens and set in a manner to enclose the whole photodetecting surface 6 in the widthwise direction seen from a plane. Therefore, the incident light passing the other area than the band-shaped lens 3 does not reach the photodetecting surface 6, making it unnecessary to coat the other area than the band-shaped lens 3 of the transparent moving plate 1 to shut light.