1. A method of deciding a measurement value for deciding a quality of the measurement value of a line width, a line interval or the like of a pattern on an object to-be-measured, comprising the steps of: acquiring a signal intensity distribution of the pattern on the object to-be-measured; detecting edge positions of the pattern from the acquired signal intensity distribution; detecting taper widths of edge parts of the pattern from the acquired signal intensity distribution; and deciding that the measurement value calculated on the basis of the detected edge positions is correct, when the detected taper widths fall within a predetermined range set beforehand, and that the measurement value calculated on the basis of the detected edge positions is defective, when the detected taper widths fall outside the predetermined range.