Different loads are applied to the skeletal muscle of at least one of an upper arm, an upper limb girdle, and a lower limb of the test subject to cause isometric muscle contraction under each load and measure the muscle activity of the skeletal muscle. Then, an upper limit of the muscle activity in a region where a magnitude of the load and an intensity of the muscle activity have a linear relationship is determined as the limit in an appropriate muscle activity range that suits the test subject. Subsequently, the muscle activity of the skeletal muscle of a test subject operating an instrument is measured, and the test subjects force for operating the instrument is evaluated based on a frequency distribution where the frequency of the muscle activity of the skeletal muscle being exercised to operate the instrument is distributed with respect to the obtained limit.
