The human elbow joint has two degree of freedom. Especially, wide range for the rotation of the forearm (pronation - supination) is attained, because of the sophisticated complexity of the human elbow joint. The elucidation of its mechanism is useful for medical evaluation and application to welfare devices. The purpose of the study is to develop the arm model that imitates the musculo -skeletal system of human elbow joint. In this paper, we made a simplified physical model with a bionic elbow joint and evaluated the mobility of the arm model by the moment arm.