PURPOSE: To accurately measure the displacement of a long material at a low cost by bringing ring-shaped objects having a constant thickness into contact with the surface of the long material, and measuring the displacement of the inner periphery of the ring-shaped object.

CONSTITUTION: Circular rings 15 made of iron are arranged to sandwich measured material 19 from above and below, and the displacements of the inner faces of the rings 15 are measured by eddy current type displacement sensors 18 installed in the rings 15. The displacements of the inner faces of the rings 15 fluctuated independently from the sensors 18 are measured, thus the production precision of rotary shafts and bearings 17 is not required to be increased. A disk 24 having a slightly smaller outer shape is fitted on one side of each ring 15, and the bearing 17 is fitted to its center. The bearing 17 is supported by an arm 14 fitted to a fulcrum 10 via a joint 16. The joint 16 has relatively large rigidity in the rotating direction of the ring 15, and the outer face of the ring 15 is closely stuck to the measured material 19 face to improve measurement precision. High production precision is not required in this method, assembling is simplified, maintenance is easy, and accurate measurement can be made at a low cost.