PURPOSE: To enable the compensation for a gap magnetic flux without the need for selecting a mutual inductor with respect to changes in the dimensions and the number of turns of a measuring coil while enabling the measurement of a magnetic field by introducing an output of the small mutual inductor into an electronic circuit which is allowed to vary amplification attenuation of a signal and finely adjusting it to compensate for the gap flux on the output thereof.

CONSTITUTION: A magnetization coil 4 is connected to series to the primary side of a small mutual inductor 10 and a magnetizing current flows therethrough with the application of a voltage from a magnetization power source 7. A secondary induced voltage of the mutual inductor 10 is applied to a differential amplifier 14 through an amplification factor adjuster 11, an amplifier 12 and an output adjuster 13. An induced voltage of a flux detection coil 3 is applied directly to the differential amplifier 14. The differential amplifier 14 acts as subtractor for determining a difference between both input voltages without amplification by setting the amplification factor at 1. Output from the differential amplifier 14 is read with an average value voltometer 8 to calculate a flux density of a sample 1. On the other hand, to obtain a magnetic field, the secondary induced voltage of the mutual inductor 10 is amplified via the amplification factor adjuster 11 and an amplifier 12 and read out with an mean voltmeter 9 led from the amplifier 12.