PURPOSE: To enable communication between a master station and all slave stations by selecting communication attributes for enabling all the stations to perform the communication and performing unification to the selected communication attributes even when the slave stations of the different communication attributes coexist.

CONSTITUTION: The master station confirms the response state of the slave station at the time of the previous communication in a response state table. Then, when the slave station responded, polling is performed to the slave station with the communication attributes of a communication speed, a communication mode and a communication code at present, etc. On the other hand, when the slave station did not respond in the response state table, the polling is performed to the slave station with the specified communication attributes. The slave station uses the specified communication attributes and informs the communicable communication attributes to the master station. Thus, the master station decides the optimum communication attributes for enabling the communication with all the slave stations and informs and sets them in the respective slave stations. Thus, even when the slave stations at different specifications coexist, the communication with all the slave stations is secured.