PURPOSE: To enable a constriction position and a constriction of a test piece to be measured accurately by emitting a laser beam which is swept in longer direction or width direction of a test piece and then obtaining a profile data over the entire length of a parallel portion of the test piece.

CONSTITUTION: A laser beam which is swept in width direction and longer direction of a test piece 1 from a laser beam source 2 is emitted to a range 1a which is expressed by a dotted line at a parallel portion of a tensile testing piece 1 and a transmission light is received by a CCD camera 3 which is installed opposingly sandwiching the test piece 1. A brightness signal for each picture element of the camera 3 is transmitted to an image operation device 4, image operation is performed, and a profile data over the entire length of the parallel portion of the test piece 1 is obtained by performing image operation. A width distribution in length direction of the parallel portion of the test piece is known from this profile data and a position (constriction position) and the width dimension (constriction) where the width of the parallel portion becomes minimum is obtained from this width distribution. This method allows an accurate constriction to be measured without affecting other tensile- testing values with a relatively simple facility.