In the bandwidth-allocating method of the present invention for PON (: Passive Optical Network), a bandwidth is allocated from an optical line terminal to each optical network unit. The optical line terminal stores a bandwidth contract table for indicating a correspondence relationship between communication flow IDs and service quality parameters, each communication flow ID identifying each communication flow between the optical line terminal and each optical network unit. The bandwidth-allocating method includes a step of transmitting a service-quality request message including the communication flow IDs and the service quality parameters from each optical network unit to the optical line terminal, and a step of the optical line terminal's updating the bandwidth contract table based on the service quality parameters, and performing the bandwidth allocation to a communication flow specified by the corresponding communication flow ID based on the bandwidth contract table.