An extended user interface compatibility confirming method has the steps of: (a) connecting an image forming apparatus and an extended user interface device to a network, the extended user interface device having an extended operation panel and sending a web service request from the extended operation panel to the image forming apparatus to perform an image forming function corresponding to the web service request; (b) requesting manufacturer information of the image forming apparatus from the image forming apparatus with SNMP protocol by the extended user interface device, and receiving the manufacturer information from the image forming apparatus by the extended user interface device; and (c) sending a web service request compatible with the manufacturer information to the image forming apparatus by the extended user interface device to inquire whether or not the image forming apparatus is capable of using the extended user interface device.