The pyrophotosensor PPS with 1.0 /spl mu/m-thickness is a device utilizing the magnetic property of a thermally-sensitive ferrimagnetic thin film with low Curie temperature. The conventional photomagnetic semiconductor using a thick film with 55 /spl mu/m-thickness has a slow response and a large size, while the PPS is characterized by a quick pulse response and a high sensitivity. When light is radiated to the PPS, the PPS absorbs light energy which causes the reluctance change to the PPS. The reluctance change can be read out by a magnetoresistive element and it is converted into voltage of which the peak value grows with an increase in illuminance. The PPS is useful as a magnetic sensor as an alternative to electric photosensors.