PURPOSE: To detect signals in such a way that the detection signals are allowed to come to equilibrium even though a semiconductor substrate is slant by containing in advance gain values which are corrected towards respective amplifiers in a memory.

CONSTITUTION: A mark 16 which is formed on a semiconductor substrate is irradiated with a charge beam or a laser beam and its irradiation makes at least four detecting means 4a-4d detect secondary electrons, reflecting electrons, light, and the like. The detecting means 4a-4d are converted into electric signals 9a-9d. They are transmitted to amplification means 5a-5d which are mounted by causing them to make a pair of two each. Then the amplification means amplify the above signals according to a given gain at a register value which is provided in the amplification means. In such a case, control operational means 7 call the optimum combination of gains of respective amplification means which are contained in advance in a memory 8 according to slanting conductions of the semiconductor substrate and then, the gains are set to each register of respective amplification means. Detection signals are thus detected by causing the signals to come to equilibrium.