A concern about fatigue has been toning up these days. Although the evaluation of muscle fatigue has been developing based on physiology lately, these are invasive ones. Muscle fatigue accompanies the change of its biomechanical characteristics in general. The authors examined the relation between muscle fatigue and biomechanical characteristics (visco-elasticity) with a biomechanical impedance measurement system non-invasively. First, it was confirmed that our system could detect the visco-elasticity of muscle with experimental models. Next, isometric contraction was applied to a subject's forearm with 10 and 15% of maximum voluntary contraction (MVC) for a certain constant period until one got fatigued. As a result, the elasticity decreased and the viscosity increased under contraction. The tendency was the opposite with no load. The amount of load did not affect the tendency but the fatiguing speed.