At the power-on timing, a device search request is sent in broadcast transmission or multicast transmission from a server side device. Device search response data is transmitted from a client side device to the server side device. At the server side device receiving the device search response data, any client side device present on the network is detected. The server side device determines a client side device whose software can be updated based on the device search response data, and transmits the required software to the client side device to request update. The client side device receives the software from the server side device to update its own software.