PURPOSE: To improve transmission efficiency by eliminating variance at the time of polling for power recovery, by performing the polling a certain time after the power source is turned off so that when the power source of a terminal station recovers, the polling is carried out automatically.

CONSTITUTION: A central control station 1 detects the power-OFF state of a terminal station from the time-out of a response from the terminal station. In this case, the log regarding the order wherein the power sources of terminals stations are turned off is stored. On the basis of the log, only one power-OFF terminal station is polled on one completion of polling, and the order is conformed to that wherein the power sources of the terminal stations are turned off. Namely, only one power-OFF terminal station 2c among power-OFF terminal stations 2a, 2b, 2c, and 2y is polled for the 1st time, and the power-OFF terminal station 2a is then polled for the 2nd time; and the terminal station 2y is further polled for the 3rd time to acheive the recoveries of the power sources.