PURPOSE: To always correct a detection error caused by temp. characteristics and change with the elapse of time to stably perform magnetic field measurement with high accuracy for a long period of time, by providing a calibration means equipped with a means for correcting an electric signal by using the ratio of each current component among detected electric signals.

CONSTITUTION: Light emitted from a light emission part 1 is passed through an optical fiber 2 and photodiodes 11, 12 and converted to electric signals by converter amplifiers 13, 14. AC components among voltage signals V1, V2 are taken out by band pass filters 18, 19 and converted to DC currents by detectors 20, 21 to be integrated by gate integrators 22, 23. By this integration, noise influence can be reduced. When the output of the integrator 23 becomes smaller than that of the integrator 22 by temp. characteristics and change with the elapse of time of a converging lens 7 and the photodiode, the ratio thereof is calculated by a division device 24 and the calculated value is multiplied by the original signal by a multiplier 25 to correct an error.