1. A length measurement system capable of figuring out a position of an edge of a pattern by finding the two-dimensional intensity distribution of a signal obtained from a patterned sample through observation by a microscope, wherein, when an edge of a pattern or the pattern in the two-dimensional intensity distribution is that of a line pattern or the line pattern itself, an image processing is carried out to average the signal intensity for a given portion of the pattern, which includes both right and left edges of the line, by specifying the number of pixels to be averaged in a direction vertical to the edges or line, that is, by specifying a smoothing parameter Sm, then another processing is carried out to calculate the edge position or line width with respect to each of Sm values that are varied sequentially, thereby figuring out the dependency of each object edge position or line width on the Sm value.