In a communication system having a network capable of converting data in a first format into data in a second format to perform communication, and a data communication apparatus capable of performing communication of the data in the first format, the data communication apparatus transmits information indicating a priority level of an image such that the network can recognize the information, when transmitting the image to the network in the first format. The network converts the data in the first format transmitted by the data communication apparatus into data in the second format, and attaches priority level data, based on the information indicating the priority level of the received image, to the data converted into the second format when performing communication.