PURPOSE: To easily and certainly measure the etching speed of a specimen, by a method wherein the timewise change of the intensity of a signal containing the information of a specimen component liberated in the ion etching of the specimen is operated and a time variable is eliminated.

CONSTITUTION: A signal containing the information of specimen components such as an Auger electron while a specimen having a know concn. gradient is subjected to ion etching and the timewise change of signal intensity thereof is electrically treated to be converted to output proportional to an etching speed. A differentiation circuit 1 operates the timewise change of the intensity Ip of the information signal of the specimen components such as the Auger signal liberated from the specimen and a division circuit 2 divides the differentiate value dIp from the differentiation circuit 1 by the intensity Ip. In this case, a differentiation circuit 3 differentiates the output of the division circuit 2 and a reversal circuit 4 reverses the code of the output of the differentiation circuit 3. In addition, change-over switches 5W8 are turned ON and OFF corresponding to the function shape of the concn. gradient of the specimen.