PURPOSE: To make it possible to measure the positions of gage marks accurately even if a test piece is twisted by providing a constitution wherein a pair of levers and a pair of encoder attaching plates are rotated with a loading shaft as the center in correspondence with the torsion of the test piece.

CONSTITUTION: A pair of levers 11 and 12 are attached to a pair of gage marks of a test piece TP. A pair of encoder attaching plates 13 and 14 are provided. The plates 13 and 14 are rotatably supported so that the plates can be rotated together with a pair of the levers 11 and 12 as a unitary body with a loading shaft 4 as the center in correspondence with the torsion of the test piece TP. The plates 13 and 14 can be relatively displaced together with a pair of the levers 11 and 12 in correspondence with the elongation of the test piece TP. Encoders 15 and 16 detect the relative displacements of the encoder attaching plates 13 and 14 and the levers 11 and 12. When the torsion is generated in the test piece TP, a pair of levers 11 and 12 and a pair of the encoder attaching plates 13 and 14 are rotated as a unitary body with the loading shaft 4 as the center in correspondence with the torsion. Therefore, excessive force does not act on the levers 11 and 12. The amounts of displacements of the upper and lower gage marks can be accurately detected. Even if the torsion of the test piece is large, the test piece is not deviated.