A data relay unit capable of relaying data communication between first and second networks interchanging different structures of data, a gateway (GW) and a method are provided. An identifier is assigned to transmission data and reception data exchanged in a GW unit. A data conversion section specifies a conversion process using reception data for generating transmission data for each transmission data piece identified by an identifier and uses a conversion table to store conversion processes. The data conversion section can generate intended transmission data based on an identifier of the transmission data to be transmitted by selecting and executing a necessary conversion process from the conversion table. Even when a method of generating transmission data is added or changed, it is only necessary to add or change a conversion process in the conversion table.