Recently, many studies on assist robots are being conducted, in which the development of a unique system is required to support aged persons, physically handicapped persons and/or caretakers. One of the representative systems is called the assist suit and is partly practiced. The assist suit is a mechatronics device which can assist physical human actions. However, the current assist suit has a few problems with respect to cost, size, weight, long-time run and soon. In this article, a fundamental study concerning a compact assist device is conducted. Where the assist device supports is one spot on the body such as a knee, an elbow and a shoulder. First of all, a simple single-axis arm is designed by using an ultrasonic motor which can generate high torque from a low velocity range. Then, a servo system, a torque control system and a passive torque control system are applied and their characteristics are evaluated. Here, the passive torque control includes a stiffness control and a compliance control.