The automatic musical instrument contains a main body provided with a sliding saddle member and a sliding operation piece. The sliding operation piece is provided with a reflecting pattern consisting of light reflecting regions and light absorbing regions. The sliding saddle member is provided with a infrared emitting diode and two phototransistors inside. When the sliding operation piece is slidably moved in contact with the sliding saddle member, infrared light emitted from the infrared emitting diode is reflected by the reflecting pattern. Then, the phototransistors receive the reflected light and output pulse signals A and B. The high speed processor built in the main body calculates the sliding speed, the sliding direction and the sliding time on the basis of the pulse signals A and B. Also, the high speed processor controls the movement of the character image displayed on the screen on the basis of one of the sliding speed, the sliding direction and the sliding time.