1. A method of measuring non-linearity of a pattern edge comprising the steps of: disposing a pattern to be measured so that a photosensitive picture element column in a light-receiving unit is substantially parallel with a pattern edge of an image of said pattern to be measured; moving said light-receiving unit or said image of the pattern to be measured so that said photosensitive picture element column in said light-receiving unit traverses said pattern edge of the image of the pattern to be measured in a manner substantially perpendicular thereto; memorizing a signal indicative of light received by said photosensitive picture element column at each of a plurality of predetermined moving distances of said light-receiving unit or said pattern to be measured; determining a pattern edge postion per each photosensitive picture element by making use of said signal indicative of the received light thus memorized; and computing a non-linearity of the pattern edge on the basis of the determined pattern edge positions.