To make efficiently and highly accurately performable a tensile test and a fatigue test by windening a part of the width of a groove for fixing a test piece.

The width of the groove 6 for fixing a strand test piece provided to a gripper has a part widened in a tension applying direction. That is, the width 51 of the groove is made wider than the width 52 thereof. Further, the shape of the end part of the test piece is molded so as to become wider than the diameter of the original test piece. Furthermore, a thermostatic tank heating only the measuring part of the test piece substantially is provided. By this constitution, the viscoelasticity of a resin impregnated strand can be measured highly accurately and efficiently not only under a room temperature condition but also under a raised temp. condition.