We propose a torque control strategy of a robot arm using ultrasonic motors. The ultrasonic motor (USM) is an actuator that has advantages such as low speed, high torque (good response time), and high-accuracy positioning without gear. These characteristics are suited for a wide range of robot arms from industrial robots to personal robots. In this report, we show the control strategy that changes the torque by changing phase of two AC voltages that applied the USM. In experiment, one of robot arm's joints becomes back-drivable, or generates any torque by the proposed control strategy