To simplify the constitution of an application on a personal computer which accesses programmable controllers connected using different kinds of network devices.

Clients 11 to 13 generate and send transmission messages in a common format, and a message management part 21 transfers the transmission messages to communication processing parts corresponding to network devices at transmission destinations. A communication processing part converts the transmission message to the message format, corresponding to the network device and transmits it via the network device. The respective clients 11 to 13 can perform processing without being aware of the network devices at the transmission destinations, and only the message management part 21 accesses a communication processing part which communicates with the network device, so that collision of message is avoided.