At least one controller in a group selects a coordinator that manages a group key to be used in common in the group from among controllers in the group in accordance with an attribute of the controllers. The selected coordinator generates a group key, performs mutual authentication with devices and the controllers in the group, and shares the generated group key with devices and controllers that have been successfully authenticated. The coordinator then generates encrypted data and authentication data by using the group key and simultaneously broadcasts a message including the encrypted data and the authentication data.