PURPOSE: To compensate the temperature drift of a pair of Hall elements having temperature coefficients with opposite polarities by using the Hall elements to detect the magnetic field generated and then amplifying the detection results by given multiplying factors before adding the amplification results to generate a feed back signal.

CONSTITUTION: In a magnetic field generator for the electron lens of an electron microscope, the magnetic field of an excited coil (L) is detected by a Hall element 4 having a positive temperature coefficient such as an InSb Hall sensor and a Hall element 5 having a negative temperature coefficient such as a GaAs Hall sensor and then the detection results are amplified with amplifiers 6 and 7 by given multiplying factors. Next, an adder adds the amplification results to generate a feed back signal. After that, the feed back signal and the signal of a setting circuit 1 are used as the inputs to a differential amplifier 2 to drive the coil (L). Owing to the above constitution, it is possible to cancel the influence of the temperature drift of the Hall elements 4 and 5 by controlling the gains of the amplifiers 6 and 7 and the adder 8. Consequently, it is possible to stabilize the magnetic field.