PURPOSE: To know accurately electron ray irradiation time wherein the influence of contamination becomes not negligible by preparing a detector detecting the strength of electron rays and a means to operate the ratio of the output value of said detector before and after lapse of a fixed time.

CONSTITUTION: The speed of generation of contamination sticking to a sample differs depending on a kind and a quantity of gas particles absorbed by the sample, a degree of vacuum around the sample, current value of the sample irradiating electron rays and whether or not using a cooling trap to check contamination while being considered to be almost fixed within the time observing the same sample. The computer 18 as illustrated puts in the values (a) and (c) expected in advance and operates the ratio I/Io and the logalithm thereof log I/I from the detection value I and Io to be got from a penetrating electron detector 11 to get the thickness (c) of the contamination generated in a time T and indicate the value thereof on the indicator 19.