Seat members 31 are held in a state of being movable with respect to first slide members 21 or second slide members 22 correspondingly. By rotating screws 39 in directions to increase the distances d between surfaces A of the first slide members 21 or the second slide members 22 and surfaces B of the seat members 31, chucks 25 are moved in directions to increase the distance between the chucks to apply pretension to a test piece correspondingly. When backlash in a force transmission system from a support part to the respective chucks 25 is eliminated, a biaxial tensile test is started.