To secure a ground speech path and to secure a communication means from a mobile station to the ground by monitoring the states of a main central device and a sub central device and automatically switching the sub and the main, when abnormality is present.

During an operation as main, when a main central device 1 becomes dysfunctional, it is judged that the central device 1 is dysfunctional in the state monitoring control part 5 of a main switching device 3. Furthermore, the information for indicating that the sub central device 2 is in a normal state, and a standby state is obtained from a sub state monitoring control part 6 and the line of the main is disconnected in the main switching device 3. At the same time, the information indicating that the central device 1 is dysfunctional is transferred from the state monitoring control part 5 to 6. A sub side switches the line from the main to the sub by the control method of pulling in the line by sub switching equipment 4, based on the information, connecting it to the sub central device 2, connecting it to a sub terminal 12, and communication between a mobile station 10 and the sub terminal 12 is enabled.