PURPOSE: To provide a device and method for measuring the magnetic field by which any error because of changes in optical power of a light source with time lapse can be eliminated and the linear relation between the outputs of a magnetic field to be measured and a sensor be improved without reducing the sensitivity against the magnetic field.

CONSTITUTION: A coil 20 around a Faraday rotor 15 generates an AC magnetic field separately other than a DC magnetic field to be measured A light beam emitted from a light source 11 passes through an optical fiber 12, lens 13 and polarization element 14, and it is given an optical rotation corresponding to the intensity of a synthesized magnetic field of the magnetic field to be measured and AC magnetic field, by means of the rotor 15, then it obtains an optical power corresponding to its intensity after passing through an analyzer 16. The light beam is converted into current by an optical detector 19 and thereafter it is sent to a lock-in amplifier 21 so as to be separated into an output element (angular frequency ω of the AC magnetic field and output element of the DC magnetic field. Then, an analog divider 22 calculates a ratio of AC element and DV element. The intensity of the magnetic field to be measured can be thus obtained based on the calculated ratio.