PURPOSE: To prevent the generation of errors occuring in the bending of a linear scale in an optical length measuring device by supporting a read head movably in such a way as to determine the microrotating center of the head at the center in the thickness of said scale.

CONSTITUTION: A linear scale 1 formed with an inscribed line belt 1a is disposed in the aperture 9a of a holding member 9, and is fixed in a groove 9b by means of rubber 10. The arm part 2' of a moving member 2 passes between dustproofing seals 11 and is extended as four extension parts 12. The points 13W16 in 4 ways in a detecting part 4 having a light emitting element 5, an index scale and a photodetector 6 on the inside surface are connected to the parts 12 by means of four linear springs 3. The bending of the scale 1 is absorbed by the springs 3, and the guide member 8 fixed to the inside surface of the detection part maintains the constant space between a scale surface 1b and the detecting part. If the point (c) of intersection of the diagonal lines of the 4 points 13W16 is beforehand made to coincide with the central line (n), the errors in detection are reduced.