A mode of a publisher or a subscriber is stored. Also, a first identifier indicating affiliation to a first group is stored. In a case of the publisher, a transmission message including ToC information, which is a list of the first identifiers and data to be transmitted, is transmitted to an outside. In a case of the subscriber, when a received message includes the first identifier, data included in the message is stored into a non-volatile memory, and when it is determined that all data in the ToC information stored in the received message are already received and that the number of publishers is less than a threshold, the stored mode is changed to the publisher.