In size measurement of a semiconductor device, profiles of a pattern formed in a resist process are determined through an exposure/development simulation in respect of individual different combinations of exposure values and focus values to form a profile matrix and scattered light intensity distributions corresponding to the individual profiles are determined through calculation to form a scattered light library, thereby forming a profile library consisting of the profile matrix and scattered light library. A scattered light intensity distribution of an actually measured pattern is compared with the scattered light intensity distributions of the scattered light library and a profile of profile matrix corresponding to a scattered light intensity distribution of scattered light library having the highest coincidence is determined as a three-dimensional shape of the actually measured pattern.