Disclosed herewith is a length measurement system, which obtains a value closer to its true one when figuring out the size and edge roughness of a pattern from a noise-included pattern image. Among plural band-like regions representing a portion around an edge in an image respectively, the system calculates the dependency of the edge point position on the image processing parameter at each of a narrow width band-like portion and a wide width band-like portion to calculate an image processing condition that calculates each measured value closer to its true value or estimates the true value itself.