PURPOSE: To perform an optically reliable measurement of a moving distance of a sensor, by a method wherein an image sensor having a plurality of picture elements is movably located in parallel to a scale plate on which marks are provided at equal intervals, and the number of the marks through which the sensor passes and the number of the picture elements before and after the sensor moves are counted.

CONSTITUTION: An image sensor 3 comprising CCD is movably located in parallel to a scale plate 1 on which marks 2 are provided at equal spaces. In case the line image of the mark 2 is projected on a j-th and a k-th picture elements from the left of the sensor 3, a right end 3R of the sensor 3 is x1=(m-k).d, where m is a total number of the picture elements, d is the width of the picture element. x2 at a right end at a time when the sensor 3 is brought to a stop after passing n pieces of the marks 2 is found in a similar manner. Thus, if the distance between the marks 2 is p, a moving distance l of the sensor 3 is equal to n.p+x2-x1, and counting of saidequality is performed by a given computing circuit. This enables the moving distance of the sensor to be found optically reliably.