PURPOSE: To identify various signal contents transmitted with an ultrasonic microphone by providing a control circuit to amplitude-modulate two optional frequencies in the unstable multivibrator of plural oscillating frequencies.

CONSTITUTION: The frequency of one of two optional unstable multivibrators 2 is, as a control signal, a carrier-frequency (fc) used when a transmitting signal is executed, and a carrier, for which the transmitting signal is executed by a modulator 4, is amplitude-modulated by a frequency (fa) oscillated by other unstable multivibrator 2. In such case for a carrier (fc) not audible to a human body and other oscillating frequency (fa), the loser side wave of the frequency component of fc-fa is generated by the amplitude modulation, while an audible frequency component is included as a 'beat,' an ultrasonic microphone is oscillated and controlled and the transmitting condition can be recognized with an ear. By selecting (fal) with a selecting switch for the oscillating frequency (fa), the 'beat' different from the one mentioned earlier of an fc-fal can be heard by the human body.