Referring to design data for a sample, a measurement region is defined at a portion in the design data which has no step in an edge of a pattern. In addition, an edge as a characteristic portion is detected from the design data, and an edge as a characteristic portion corresponding to the characteristic portion of the design data is detected from a secondary electron image. Then, the measurement region is positioned and located in a secondary electron image based on a positional relationship between the edge of the design data and the edge of the secondary electron image. A width of the pattern is measured on the basis of a distance between the two edges included in the measurement region thus located.