PURPOSE: To control a transmission power of an on-road transmission station over a wide temperature range to a prescribed level by detecting part of the transmission power so as to monitor the transmission power and correcting an amplification factor of a power amplifier section with a data in response to the operating temperature.

CONSTITUTION: A high frequency signal is inputted from an input terminal 1 and amplified to a prescribed power by a power amplifier 2. The amplified power is distributed to a power outputted to a transmission antenna 4 and a control power by a directional coupler 3. The control power is detected by a detection section 5 and converted into a DC voltage. Moreover, a temperature detection section 13 detects the temperature and a detection signal is converted into a DC voltage. Then the power amplifier 2 is controlled by a control voltage resulting from two DC voltage data calculated by a CPU 8 and obtained by a PWM generator. Thus, not only the temperature of the power amplifier section but also the temperature fluctuation of other control loop system is absorbed and the fluctuation of the transmission power due to a temperature change is reduced.