PURPOSE: To attain the communication between nodes each having no 3-layer while enabling the communication between nodes each having 3-layer by the same method as a conventional method by connecting two networks in which the address system of MAC sub-layer differs from each other.

CONSTITUTION: After a relay function is discriminated at an LSAP address in an LLC sub layer, an MAC address is obtained by either method of introduction from address information added at an application layer or definite decision from the LSAP address and the address is relayed. Moreover, after the relay function is discriminated by the LSAP address in the LLC sub-layer to eliminate and add the LSAP address. Thus, while the communication between nodes having a network layer is implemented by a conventional method, the communication between nodes each having no network layer arranged in different networks is attained.