In a system in which a server and a client to which a device is connected communicate, the server can execute processes based on the result of checking device operation. The POS terminal of a network system has a device control function for controlling the device, and a diagnostic function for checking operation of the device. To check the operation of the device connected to the client, a maintenance server sends a control command instructing starting the diagnostic function to the POS terminal. The POS terminal stops the device control function if the device control function is executing, starts the diagnostic function, checks operation of the device with the diagnostic function, and returns the result of the checked operation to the maintenance server.