Each of terminal devices 3 belongs to one of groups, starts up at a startup time, and performs terminal processing within a communication time after an offset time is lapsed. A control device 2 uses a sum of the communication times of the terminal devices 3 belonging to the same group as the communication time of the group. The offset time is calculated for terminal devices 3 belonging to another group which performs the terminal processing subsequently to the terminal devices 3 belonging to the previous group, based on the communication time of the previous group. A next startup time for each terminal device 3 is determined based on the communication time and the offset time, and it is set to the terminal device 3.
