To prevent a noise radio wave from being radiated by detecting a signal out of a prescribed band from the output signal of an amplifier and controlling a transmission output circuit according to the detected result.

The signal of an intermediate frequency inputted to the transmitter is passed through a band pass filter 1, amplified by an amplifier 2 and distributed into two by a distributor 3. One distributed signal is made into high frequency signal by a frequency converter 4, passed through a transmission output circuit 5 to a band pass filter 6 and radiated from the transmitter later. Concerning the other signal distributed by the distributor 3, the signal component of a band widened by amplifying a noise is passed by a band block filter 7 and that passed signal is detected by a detector 8. The voltage and current of the transmission output circuit 5 are controlled through a control circuit 9 by the detected signal and when the presence of the passed signal is detected by the detector 8, since the output wave of the transmission output circuit 5 is suppressed, no transmission signal is radiated from the transmitter.