To enable communication in accordance with a RPC through a firewall in using email, the server-side communication device has a mail transmitting and receiving unit and an information converting unit for retrieving instruction information from an email received with the mail transmitting and receiving unit, for outputting the instruction information to a RPC server, for converting response information corresponding to the instruction information into a return email to a client-side communication device 7, in which the mail transmitting and receiving unit transmits the return email output with the information converting unit to the client-side communication device. On the other hand, the client-side communication device has a mail transmitting and receiving unit and an information converting unit for outputting the email to the mail transmitting and receiving unit after converting the instruction information transmitted to the server-side communication device into the email, for retrieving the response information corresponding to the instruction information from the received email, for outputting the response information to a RPC client unit, in which the mail transmitting and receiving unit transmits the email output with the information converting unit to the server-side communication device.