PURPOSE: To transmit a continuous modulated signal stably by detecting the temperature in a transmitter through a temperature detecting means and then controlling the output voltage from a power controller to the driving amplifier of the transmitter when the temperature in the transmitter rises above a prescribed temperature.

CONSTITUTION: A transmitting circuit 20 is provided with an automatic power controller 30 relatively, and the voltage at an input terminal B is stabilized by a constant voltage generating circuit 31 and supplied to a DC amplifier 122. A modulated signal from an input terminal A is amplified by amplifiers 21 and 22, passed through a strip line 23, and transmitted from an antenna 26 through an isolator 24 and a filter 25. When the temperature in a transmitter rises abnormally up to temperature t2, a temperature detecting circuit 32 supplies its output to the base of a transistor 321, which turns on to supply a constant voltage to a DC amplifier 122. Consequently, the controller 30 supplies the output voltage of a control circuit 12 to a driving amplifier 21, thereby transmitting the modulated signal stably and successively with a lower level than a transmission output at the prescribed temperature t2 and below without interrupting the transmission of the modulated signal.