There is provided a data processing method of a client terminal that communicates with a server, including: receiving a data request message for requesting data of a USB device connected to the client terminal, from the server, the data request message including information about a size of data that is to be read from the USB device; acquiring data corresponding to the size of data included in the data request message, from the USB device; deciding a size of data that is able to be additionally transmitted from the USB device, according to operation states of other USB devices connected to the client terminal; and transmitting a data transmission completion message to the server, wherein the data transmission completion message includes information about the size of data that is able to be additionally transmitted, and the data acquired from the USB device.