COMMUNICATION SYSTEM

PROBLEM TO BE SOLVED: To provide a communication system capable of quickly and easily changing the communicating party of a slave unit without the need of changing the communicating party in the slave unit at a slave unit installed place at the time of fault of a master unit. SOLUTION: For this communication system, a present ID, a slave ID and a spare ID are held in a master unit 1, a present ID, a master ID and a spare ID are held in the slave unit 2 and a present ID, a slave ID and a spare ID are held in a spare master unit 4. Then, at the time of the fault of the master unit 1, the slave unit 2 transmits a sensor change report c to the spare master unit which corresponds to the spare ID held by the slave unit 2. When the spare master unit 4 which receives the sensor change report c transmits a confirmation response d including the spare ID held by the master unit 4 to the slave unit 2, the slave unit 2 changes the master ID and the spare ID and changes the communicating party.