A dimension measuring apparatus used for measuring a dimension of a semiconductor device having a first pattern of repeated structure and a second pattern that is linear and formed on the first pattern to extend over the repeated structure. The invention includes a shape information acquisition unit which acquires information on a shape of the first pattern; a width value acquisition unit which acquires a width value of each portion of the second pattern; an analytic area setting unit, which sets a plurality of analytic areas on the second pattern; and a dimension determining unit, which extracts, for each of the set analytic areas, width values of portions included in the analytic area, and uses the extracted width values to determine a dimension of the second pattern at portions overlapping the first pattern.