PURPOSE: To measure a variation in length to a reference value to simplify a positioning mechanism and to make easy to operate by projecting a light only to both edge parts of an object to be measured using an optical system that is inverted and changes the direction.

CONSTITUTION: A light P of light beams P, Q at both ends of a line beam emitted from a projector 1 is shielded partly at an edge part 7a of one side of a material 7 to be measured, and is received at a light receiving part 2 as a light beam P. Further, the light beam Q is reflected by an optical system 3 and is shielded at an edge part 7b of the other side of the material 7 to be measured to produce a light beam q, and a line light beam having a width specified by light beams p, q is received with the light receiving device 2 to determine the width of the light flux. It is possible to measure a length by setting a reference length lo of the material 7 to be measured to measure the amount of the variation.