A network system including a customer replaceable unit (CRU) having an excellent security function for operation information which system can realize improvement of the use efficiency of operation information for making the CRU operate, and reduction in costs, is provided. An apparatus main body of a multifunction printer, a server, a personal computer, etc. are connected to a network of a network system, and a CRU is attached to the apparatus main body so as to be detachable. Operation information necessary for making the CRU operate is stored in the server. The apparatus main body obtains operation information of the CRU from the server via the network and stores the obtained operation information into a buffer memory, and causes the CRU to operate based on the operation information. The operation information stored in the buffer memory is erased when communication between the apparatus main body and the CRU ends.