PURPOSE: To readily equalize the amplitude without requiring the measurement of equivalent amount be equivalent and to improve the stability of an output signal level against temperature change.

CONSTITUTION: The type of transmission cable, cable length, and ambient temperature information as an address are inputted in memories 7 and 10. D/A converters 6 and 9 input the output data of the memories 7 and 10, outputting control voltage to input and gain variable amplifier 8 of a pin diode 5. The gain variable amplifier 8 corrects the fluctuation of the output signal of a buffer amplifier 3 generated by the change of the amplitude equivalent amount and the ambient temperature based on the control voltage of the memory 10.