1. A method for measuring a periodic line and space pattern formed on a specimen, comprising the steps of: obtaining a specimen image at a magnification such as to include a plurality of line and space patterns by using a scanning electron microscope; differentiating an obtained image; calculating auto-correlation values of a differentiated image; and figuring out at least one of an average line width, an average space width, and an average pitch width of the above-mentioned line and space patterns from peak intervals of the above-mentioned auto-correlation values.