In a connection device 2, a limitation step for limiting operations receivable by an operation unit 15 of an image processing apparatus 1 on a step-by-step basis is preset. When the connection device 2 is connected to the image processing apparatus 1, limitation data corresponding to the limitation step is inputted to the image processing apparatus 1 from the connection device 2. When the connection device 2 is connected, the image processing apparatus 1 displays, on a display unit 14, an operation screen that is associated with the limitation data in advance. A special user such as a serviceman conducts special work concerning an operation of the image processing apparatus 1 by utilizing the displayed operation screen. The operation screen necessary for special work is not displayed when the connection device 2 is not connected. A general user who does not use the connection device 2 cannot conduct special work.