A facsimile communication system which is capable of selectively performing encrypted communication or non-encrypt communication according to the intention of a user. A transmitting-side digital multifunction machine inputs the port address of a receiving-side digital multifunction machine according to a user operation, and designates encrypted communication for transmitting IFP packets of image information, via an IP network, after encrypting the IFP packets, or non-encrypt communication for transmitting IFP packets of image information, via the IP network, without encrypting the IFP packets. The receiving-side digital multifunction machine determines whether the input port address is for an encrypted communication port or a non-encrypt communication port, and sends a response containing an associated port number to the transmitting-side digital multifunction machine. The transmitting-side digital facsimile apparatus establishes connection to the port with the port number, and performs the designated communication.