To efficiently execute mutual communication by connecting between a client machine and a host machine of different communication protocols through a data processor with a protocol converter.

In the case of executing transmission and reception between a client machine 1 and a host machine 8, data is transmitted from the client l to the data processor 4. This data is analyzed by a client transmission/ reception means 41 and sent to a connecting destination sorting means 44. From the result of analyzing, the means 44 retrieves a client storing part 51 to decide a host machine to connect, supplies data for a host transmission/reception means 47 to edit data for a communication protocol C with the host machine 8 and then transmits data to the host machine 8. In the case of transmitting data from the host machine 8 to the client machine 1, data is similarly sent to the means 41 and data is edited for a communication protocol A to be transmitted to the client machine 1.