To perform exact measurement without making a testee receive great load by preventing the body movement of the testee in the expanding direction of legs or toward the head in the case of measurement.

This instrument has a supporting means for supporting the body of the testee and the strength of the legs in the case of action, a stepping power receiving means for the testee sitting on the supporting means to move with the expanding direction of the legs as an axis, a driving means for accelerating and decelerating/driving the stepping power receiving means back and forth in the direction of that axis, a load measuring means for measuring the load operated upon the soles of the testee, a measuring means for measuring the position, speed or acceleration of the leg expansion of the testee and a control means for controlling the driving means. The control means controls the driving means for driving the stepping power receiving means so that the load measured by the load measuring means can be preset fixed power in the leg expanding action of the testee. Further, the instrument has a holding means for holding the measuring posture of the testee.