A header processing unit includes a key information supply unit that manages MAC addresses and MAC key information having a short bit length that corresponds to the addresses, and decides key information corresponding to a source MAC address and a destination MAC address of an input frame. A destination decision unit performs destination decision processing based on corresponding key information instead of destination MAC addresses. A flow decision unit decides a flow based on key information corresponding to a destination MAC address and a source MAC address. Since the key information supply unit does not need to set plural identical MAC addresses, the header processing unit becomes inexpensive. Thus, this invention achieves an inexpensive switch that performs output decision processing and flow decision processing.