A transmission signal which is output from a power amplifier is rectified by a first rectifying circuit and is then input to a first transistor of a voltage-to-current converting circuit, while the reference voltage is output from a second rectifying circuit and is then input to a second transistor of the voltage-to-current converting circuit. The output current of the first transistor is subtracted from the output current of the second transistor via a first current-mirror circuit, and a current that is proportional to the output voltage of the power amplifier is caused to flow to a two-terminal p-n junction electronic device. Then, a voltage that is proportional to the logarithm of the current is output from across the p-n junction electronic device.