We have developed a unicycle robot that consists of a body, a disk driven by a dc-motor to stabilized its posture, and a wheel driven by another dc-motor to move the body forward. The robot has only two motors and there is no direct force to change the yaw direction of the robot. For changing the yaw direction, we use sinusoidal inputs for the disk and for the wheel. The present paper shows that an average substantial input for the yaw direction depends on the phase difference of the two sinusoidal inputs, and then proposes a control method for changing the yaw direction from an initial value to zero by modifying the phase difference. To confirm the efficiency of the proposed controller, simulations are conducted under two conditions: the one is that the wheel speed is zero, and the other is that the wheel speed has a constant velue. Under the first condition, the robot moves back and forth around an initial point and gradually changes the yaw direction. Under the second condition, the robot moves forward at a constant speed while its yaw direction gradually changes.