In order to provide a pattern dimension measurement method with a small measured error and excellent reproducibility even though defocus occurs and a charged particle beam microscope used in the same, in a method for applying a charged particle beam to a specimen formed with a pattern to measure a pattern dimension from a signal intensity distribution of signal charged particles from the specimen, edge index positions (X1) and (X2) on the right and left of the maximum point of signal intensity corresponding to a pattern edge are calculated by a threshold method, and a pattern edge position (Xe) is found from a mean value between the positions. Thus, it is possible to reduce the influence of defocus on the pattern edge position (Xe).