PURPOSE: To measure a magnetic field at an extremely small part by obtaining a closed magnetic circuit which consists of permanent magnets and a yoke of a material with high magnetic permeability and has a notch part, and applying a magnetic field to be detected to the notch part in detecting variation in magnetic flux density.

CONSTITUTION: The 1st and 2nd permanent magnets are fitted to the inside of a yoke 13 having large magnetic permeability to form a closed magnetic circuit. The magnetized area 6 of a rotating disk 5 for a fine magnetic field is inserted freely rotatably into the gap of the notch part 14 of the yoke 13. Then, lines of magnetic force passing through the gap of the notch 14 increase and decrease in the direction of lines of magnetic force by the magnetized area 6 of the rotating disk 5. For this purpose, a probe 18 for measuring a magnetic field is inserted between said two magnets 11 and 12, so that variation in the magnetic field in the magnetized area 6 is obtained between the permanent magnets 11 and 12 while expanded in terms of size.