PURPOSE: To increase the response, by using one frequency in a voice band assigned to each slave station as a confirming signal for the production of transmission with priority.

CONSTITUTION: A master station M transmits a request RQ for the data collection of a slave station R1. After the reception of the response signal RP, a request AR is transmitted to all the slave stations to confirm if the items with priority is generated at each slave station. A slave station receiving the request AR transmits one frequency in the voice band assigned to the master M as the response signal within a specified time, if the items with priority is produced in the station itselt. The master station M identifies the production of the transmission item with priority in a slave station with a received signal, transmits the request RQ to the coresponding slave station and performed data collection.