A rule for transferring data over a communication network is changed for each communication apparatus without spending time and cost. In a communication system in which a plurality of communication apparatuses are connected to a network, at least one of the plurality of communication apparatuses includes rule storage means for storing a plurality of transfer rules for transferring data over the network, transfer means for transferring data incoming over the network in accordance with one of the plurality of transfer rules, and rule switching means for switching one transfer rule to another transfer rule when the communication apparatus receives a notification from the outside.