To enable the allocation control of a communication line even when a control line is unavailable by making a radio station automatically switch to an alternate control line that is previously prepared and has a different frequency and performing the allocation control of the communication line again when a dedicated control line becomes unavailable due to an external failure, etc.

A calling station 1 designates an available communication line 4 with a control line 3 and makes a communication request for a called station 2. The station 2 responds the propriety of communication through the designated communication line 4 in response to the communication request from the station 1 with the line 3. The station 1 receiving the response performs a communication operation with the station 2 performing the response. When the lines 3 become unavailable due to an external failure, etc., at the time of making a communication request from the station 1 to the called station or making response from the station 2 to the station 1, the station 1 automatically switches to an alternate control line 5 and makes a communication request again because it can not receive the response from the station 2.