PURPOSE: To enable control of a focal length without the aid of a mechanical method by using a Faraday effect that can be seen with a transparent magnetic material.

CONSTITUTION: A titled mechanism consists of a polarizing plate 1, a quarter- wave plate 2, a lens 3 formed of a transparent magnetic material and an exciting coil 4. The light made incident thereto from the left side is polarized to linearly polarized light by the plate 1 and is thereafter polarized to circularly polarized light by the plate 2. The refractive index of the lens 3 for the circularly polarized light is controlled by the coil 4. The focal position is consequently controlled without the aid of a mechanical method.