PURPOSE: To detect the discrepancy of information by a slave station, and to easily execute a recovery processing, by transmitting periodically the code of a route information pattern which is being used, to the slave station from a master station.

CONSTITUTION: The master station transmits periodically a route information code which is being used at present, to the slave station. When the code is received, the slave station collates it with a code which has been stored, and when they coincide, the communication is continued by its route information. As the result of said collation, if the discrepancy is detected continuously several times or more, it is decided that the route information is changed, and a recovery processing is executed. Also, when the discrepancy is not detected continuously, the received discrepant code is regarded as an error, and the communication is continued by the route information pattern of the stored code.