PURPOSE: To prevent generation of a fault by applying on/off control to a power supply of a driver circuit of a transmitter depending on the presence of a fault of a modulator so as to send a carrier at no modulation thereby preventing the overload operating state of a transmitter.

CONSTITUTION: The carrier is modulated at the normal transmission state. Since an inverted input of a comparator 7 goes to a high level, the output of the comparator 7 goes to '0' and an output of an AND circuit 8 goes to '0' independently of the presence of a transmission output detection signal. Thus, a switching transistor (TR) 10 is turned on and a power is applied to a driver stage. If a fault takes place, that is, a modulation signal is '0', (no modulation), and a transmission output detection signal goes to logical '1', since both the outputs of comparators 6, 7 are logical 1, the output of the AND circuit 8 goes to logical '1'. As a result, the switching TR 10 is turned off, the supply power of the driver stage is interrupted and the transmission is stopped.