AUTHENTICATION METHOD

Power usage of each of one or more devices belonging to a group is collected at prescribed intervals, and an authentication request including group identification information for identification of the group to be authenticated is received from a terminal. Upon receipt of the authentication request, a device as a manipulation target in the group and details of a manipulation on the device are determined, and notification is provided to the terminal. Whether the device designated by the notification is manipulated is judged on the basis of a change in the power usage of each of the devices belonging to the group. If the device designated by the notification is judged to be manipulated, authentication information is transmitted to the terminal.