PURPOSE: To provide a photomagnetic field sensor which enables the manufacturing thereof at a lower production cost with a smaller size and a less weight while allowing the detection of a lightening surge with high insulating property.

CONSTITUTION: Light emitted from a light source 1 is passing through an optical fiber 2 and then. admitted into a RIG (magnetic garnet) film 5 passing through a lens 3 and a total reflection prism 4. The light passes through the RIG film 5 receiving an intensity modulation according to the intensity of a magnetic field and moreover. is deflected with a total reflection prism 6 to be condensed to an optical fiber 8 with a lens 7. Then, the light is introduced into a photodetector 9 with the optical fiber 8 and undergoes photoelectric conversion.