An object of the present invention is to suppress measurement errors caused by the fact that the shrink amount due to scan of an electron beam differs pattern by pattern.To accomplish this object, according to the invention, functions indicative of a process of change of pattern dimension when the electron beam is irradiated on a sample are prepared in respect of the kinds of sample patterns, and dimension values of a particular pattern measured by scanning the electron beam on the particular pattern are fitted to a function prepared for the particular pattern to calculate a dimension of the particular pattern before it changes.