Abstract: A reference line mark for an optical extensometer of the invention has a mark main body with a boundary shape, in which a reflectance of light is different from other portions. The boundary shape is arranged such that when the mark main body is integrated in a direction perpendicular to the deformation direction under the condition that the mark is attached to a test piece, results of the integration constitute an isosceles triangle having a base in a deformation direction of the test piece. Thus, data points for obtaining edge portions of the mark can be increased, and error-free approximation can be available. Elongation of the test piece can be measured with high accuracy at high computing speed.