PURPOSE: To keep a mobile communication even with the presence of an interfering radio wave or the like by adding a interfering radio wave detector and a constant period generator controlling an idle signal transmitter so as to transmit an idle line signal at each prescribed period when the interfering radio wave is detected to a fixed station.

CONSTITUTION: The idle line signal transmitter 1 transmits an idle line signal to whole mobile bodies when no signal comes from a mobile body. A signal detecto 2 supervises a common line, and stops the operation of the transmitter 1 when the signal transmission of the mobile body is detected. An interfering radio wave detector 3 discriminates it as an interfering radio wave when the incoming radio wave is only the radio wave and no signal exists. The prescribed period generator 4 controls the idle line signal transmitter 1 when the detected output of a interfering radio wave is received from the interfering radio wave detector 3 and transmits the idle line signal at each prescribed period. Thus, even if a interfering radio wave exists, the mobile body having excellent D/U can at least be kept to the state possible for communication.