A method is disclosed including: measuring sending amounts of packet data that are input from each of ports providing minimum bandwidth guarantee type and best-effort type communication service; calculating statistical information corresponding to an accounting subject time period of the sending amounts of the packet data of the best-effort bandwidth of each of the ports, based on the result of the measurement; changing a value of a peak information rate of a port sending out more of the packet data at a contracted communication rate to be low based on the statistical information; and impartially allocating the best-effort bandwidth to each of the ports based on the value of the peak information rate and a minimum bandwidth guarantee rate.