To keep a state under communication without disconnecting the radio channel of a transmission/reception controller at the previously set destination of switching by providing an interference wave detecting means from the outside to a channel under communication and switching the channel from the transmission/reception controller under operation to a reserve transmission/ reception controller when any interference wave is detected.

When the interference wave is detected from a controlled transmitter 7 receiver using a frequency f2, all the channels from 1 to 6 using the frequency f2 are turned into state under interference. Switching to the reserve transmitter/receiver predetermining a time slot to allocate a communication channel through a time division multiplex system is not performed but present zone channel switching or other zone channel switching is executed. When any space does not exist in the channel at the destination of switching, a talk end waiting state is provided but a control channel executes switching to the correspondent slot of reserve transmitter/receiver and a speech channel continues communication without disconnecting the channel.