A transmitter, a communication system, and a communication method of the same capable of realizing communication of more than a set bandwidth with respect to packet communication with a bandwidth guarantee set therefor by using the set bandwidth as the minimum guaranteed bandwidth, wherein when a transmitter transmits packet data, it decides whether or not packet communication guarantees a bandwidth based on a guaranteed bandwidth set in advance and, when guaranteeing the bandwidth, adds a first identifier to the packet data, while when not guaranteeing the bandwidth, adds a second identifier to the packet data and transmits the same to the network and wherein a relay unit connected to the network decides, in accordance with identifiers registered in advance and guaranteed bandwidths corresponding to same, whether or not the received packet data are bandwidth guaranteed, adds the first or second identifier to the packet data in accordance with the result of the decision, and transfers them to the next relay unit or a receiver side via the network.