A first controller generates a first group key, executes first mutual authentication with devices within a group, and shares the first group key with the devices that have succeeded in first mutual authentication. When a second controller joins the group, the first controller decides a coordinator that manages a group key used in common in the group. The first controller executes second mutual authentication with the coordinator, and shares the first group key with the coordinator when the second mutual authentication is successful. The coordinator performs encrypted communication within the group using the first group key, generates a second group key when valid time of the first group key is equal to or smaller than a predetermined value, executes third mutual authentication with the devices and a third controller, and updates the first group key of the devices and the third controller that have succeeded in the third authentication.