A method for transferring data between electronic filing systems via a communications network and using facsimile communications protocol is disclosed. The method includes the step of adding codes corresponding to non-standardized terminal capability information into a session start command (CSS) of a facsimile communication protocol and sending the CSS to a receiving end electronic filing system. The non-standardized terminal capability information includes a code indicating that the sending system is an electronic filing system and the memory capacity of the sending system, etc.The method further includes the step of adding header information at the top of the communication data to be transmitted. The header information includes an identifier identifying whether or not the data to be transmitted corresponds to an image; if it corresponds to an image, the header information further includes the size and line density of the image, the compression method for the image, etc. In the receiving end electronic filing system, the header information is decoded and the receiving end electronic filing system is controlled to properly reconstruct the received information in accordance with the decoded header information. This method makes it possible to transmit specific images such as large size images or color image and code information, which otherwise cannot be transferred with facsimile communications protocol.