An aberration of a projection exposing apparatus system is measured from the difference between widths of both-end lines even in a fine pattern having a pattern line width of less than 0.2 .mu.m. A resist film on a substrate is exposed to light, using a mask having a line-and-space (LS) pattern. This exposure is repeated plural times at plural exposures, so as to obtain, at the respective exposures, the differences between the widths of the both-end lines of the line-and-space pattern. The difference between the widths of the both-end lines at a standard (optimal) exposure is estimated from the relationship between the differences between the widths of the both-end lines and the exposures, so as to obtain an aberration at the standard exposure.