Abstract: Facsimile devices having no SIP function or operating based on different protocols are allowed to easily perform good facsimile communication over a SIP network, only with a telephone number input. A SIP adapter is connected to a facsimile device and enables the facsimile device to perform direct facsimile communication with the other facsimile device. When inputting a telephone number from the facsimile device, the SIP adapter sends the telephone number to the SIP server and acquires the IP address of another SIP adapter connected to the other facsimile device. After establishing a session between the SIP adapters, the SIP adapter acquires the IP address of the other facsimile device and transmits facsimile image data to the other facsimile device.