Easily and correctly measuring a dimension of a pattern of a photomask or of an OPC pattern of the photomask.A pattern measurement method of the present invention includes steps of obtaining both a standard pattern corresponding to a predetermined pattern and a measurement point specified in advance; setting a measurement area so that it includes two straight line segments on both sides of the measurement point among outlines of the standard pattern; and measuring a dimension between two contours of the scanned image of the predetermined pattern in the measurement area by superposing the measurement area on the scanned image of the predetermined pattern. The measurement area is set so as not to include portions near corner portions connected to two line segments.