A coexistence controller which performs management and control of time division for coexistence, is provided in a power line communication modem which has a function as a QoS controller for a QoS type communication system and a power line communication modem for a best-effort type communication system. In the QoS type communication system, by controlling allocation of a communication band in accordance with priorities given to the communication systems by cooperation of the coexistence controller and the QoS controller, the power line communication systems having different schemes can coexist.