PURPOSE: To facilitate magnetic domain observation by making the magnetic field strength of a first gap weaker than that of a second gap and setting the optimum position of an objective diaphragm within the range of the second gap.

CONSTITUTION: A magnetic pole 5 surrounding a sample 7 is placed in the middle position between magnetic pole pieces 4, 9. An objective forward magnetic field 20 is formed in front of the sample 7 and an objective backward magnetic field 21 is formed in the rear of the sample 7. At that time, the magnetic field strength of a first gap G1 is made weaker than that of a second gap G2 by adjusting a magnetic pole shape around the first gap G1 and/or the gap length and the like of the first gap G1 to form a weak lens in front of an objective lens and to radiate a beam to cased to some extent on the sample 7. An objective diaphragm 10 is moved within the range of the second gap G2, so that the observation of a magnetic domain structure on the basis of Foucault mode may be performed.