To correctly measure distortion/setting characteristics of an elastic modulus of a plastic material.

In executing a tensile test to the plastic material at a tensile tester, a plurality of marks 2a-2g are provided via an arbitrary interval to a parallel part 11 of a sample piece 1 of the plastic material to be tested. A sheet thickness of the (mark) parts 2a-2b is measured during the tensile test. A region where only a specific region of the parallel part 11 of the test piece 1 extends is judged from a relationship between an extension of the sample piece 1 and the sheet thickness. Distortion/setting characteristic of the elastic modulus of the plastic material are obtained with the user of data of the judged region.