PURPOSE: To make communication with a local area network without conversion of a protocol at a protocol transparent means in the case of the same communication protocol by providing an address registration means and an address retrieval means.

CONSTITUTION: A registered address is registered in an idle area of an address table 10. A line communication section 2 receives a data from a device 1 in the transmission and an address retrieval section 8 retrieves the address of a communication opposite party by a table 10. When the address is not found in the table 10, a protocol conversion section 5 converts the protocol into a packet. Communication control is implemented by an OSI high layer 3. When the address is found out in the table 10, a protocol transparent section 7 does not make protocol conversion but the protocol is composed into a packet. An OSI low order layer 4 makes communication control to send a packet in a local area network. The similar operation is implemented in the reception. Thus, no protocol conversion is implemented in the communication between devices using a same communication protocol and change in the communication data or transmission delay is prevented.