Communication device, train network system, and network setting method

A communication device operating as a hub in a train network system that includes a plurality of hubs installed in train vehicles, each of the plurality of hubs belonging to both a trunk network in which a signal transmission range is not limited and a branch network in which a signal transmission range is limited, and adding, to a signal to be transmitted to a hub belonging to the same branch network as it does, network type information indicating whether the signal is a trunk network signal or a branch network signal. The communication device transmitting branch network information including at least identification information on a branch network to which it belongs at a predetermined timing, and based on branch network information received from an adjacent communication device, determining whether or not to add the network type information to a signal to be transmitted to the adjacent communication device.