PURPOSE: To correctly manage a transmitting/receiving operation in a radio high frequency part by detecting voltage to be actually supplied to the radio high frequency part through a monitoring cable in a modem part.

CONSTITUTION: DC voltage V is supplied from a modem part 1 to a radio high frequency part 2 using only one signal cable 3. On the other hand, the other signal cable 4 connects a resistance R for making DC impedance into high impedance to the cable 4 itself, and after that the cable 4 is made into the monitoring cable for returning actual voltage V' supplied to the radio high frequency part 2 to the modem part 1 as it is. Consequently, the actual supply voltage V' can be detected in the modem part 1 by the monitoring cable 4, and the actual supply voltage V' can be set to original voltage by controlling the supply voltage V based on the detected supply voltage V'. Thus, the radio high frequency part 2 can be managed from a remote place.