PURPOSE: To actually measure the incident angle of charge beam in accuracy necessary for correction, by altering the height of one of two marks by known quantity and measuring the dimensions between both of two marks before and after alteration by the charge beam.

CONSTITUTION: The mark M2 among the marks M1, M2 on the surface 13a of a sample is altered to a position M2 (M1M2=M1M'2) by a known height Z to measure the dimensions L, L' between the marks by charge beam 12. When there is an incident angle (i), the mark M'2 is measured by charge beam 12' and, therefore, supposing that the mark M'2 is present at the position M"2 on the surface of the regular sample, measurement is performed and the dimension between the marks M1, M"2 becomes (l). An incident angle can be calculated from the dimension (L'Wl) and the known height Z by geometric calculation. Since the measuring accuracy of the dimension between the marks can be set to 0.02μm or less and that of the incident angle can be set to 1m radian or less, if the falling of an optical axis is effectively corrected on the basis of both accuracies, the shift of an irradiation position is suppressed to 0.005μm or less in such a case that the focal distance up to the surface of the sample is 30mm and the up-and-down movement on the surface of the sample is 5μm.