1. A sample dimension measuring method for scanning an electron beam on a sample and measuring dimensions of patterns formed on said sample on the basis of detection of electrons emitted from the scanned site, wherein functions indicative of a transition of changes in dimension of said patterns when the electron beam is irradiated on said sample are prepared in accordance with the types of said patterns, and dimension values of a specified pattern measured by scanning said pattern with said electron beam are fitted to a function prepared for said specified pattern to calculate a dimension of said specified pattern before it changes.