PURPOSE: To improve the measuring accuracy by dividing and differentiating the size signals.

CONSTITUTION: The size signals 4, which are generated by the scanning operation of such a pattern 1 in the direction of arrow 3 with electron beams as is formed in submicron unit on a substrate 2 and as is made of chromium or silicone oxide film, are suitably divided in an X (or vertical) direction so that the maxiumum or minimum is sampled in each divided section. After the division into one signal waveform by the maximum only and into the other signal waveform by the minimum only, the respective signal waveforms thus divided are differentiated to take out the respective peak values of the differentiated waveforms, from which the respective center values are calculated to determine the size of the pattern 1. Thus, even if the S/N ratio of the signals 4 is not good, the correct signals can be taken out, and the end portions of the pattern 1 can be properly grasped thereby to attain the desired object.