PURPOSE: To take a measurement with high sensitivity from a low magnetic field to a high magnetic field by providing a Faraday element having a cooling means which is made of a ferromagnetic garnet film increasing in saturation magnetic field at room temperature and performs cooling to below the room temperature.

CONSTITUTION: The magnetooptic sensor has a polarizer 10, the Faraday element 11, and an analyzer 12 arranged linearly and also has lenses 9 and 9 arranged at both ends of those optical components, etc., and is housed in an aluminum-made container 16. A cavity is formed in the container 16 and liquid nitrogen is injected to cool the element 11. The container 16 constitutes a sensor head covered with styrofoam to maintain heat insulation. This sensor head is put in an air-core solenoid. An optical fiber 8 for making light incident from an LED 7 is connected to one end between lenses 9 and 9 at both ends of the magnetooptic sensor. The lens 9 at the other end of the magnetooptic sensor makes light incident on a photodetecting element 13 through an optical fiber and the light is converted by the element 13 into a current, which is inputted to a following arithmetic circuit 14.