PURPOSE: To make the time to response logically shortest by obtaining a regular A&B form simultaneously with the start of transmission of ultrasonic waves after a push-on switch is depressed and controlling a circuit with electronic circuit parts such as a capacitor or the like or a mechanical switch or the like so that one of codes A and B which has a shorter interval of time is transmitted first and transmitting the A&B code surely in order of A→B→A (A<B) to eliminate a waste of time in the initial stage of transmission.

CONSTITUTION: When a voltage is impressed to a multivibrator circuit consisting of resistances 4, 7, 10, and 12, capacitors 6 and 11, and NPN transistors TRs 5 and 13, the circuit starts oscillation. This multivibrator circuit starts always oscillation by turning-on of the NPN TR13 because of the effect of a capacitor 8 and a resistance 9 when a power source voltage is impressed to this circuit. At this time, the capacitor 11 is charged reversely by the resistance 10; and when its potential exceeds a voltage VBE of the NPN TR, the NPN TR5 is turned on and the NPN TR13 is turned off in a moment. Then, the capacitor 6 is charged reversely by the resistance 7. If the time of this reverse charging is matched to the time of one of codes A and B of the A&B code which has a shorter time, the signal transmission is started with a shorter pulse interval.