Provided is a quality-of-service (QoS)-guaranteed video stream method. The QoS-guaranteed video stream method includes transmitting, by a transmitting server, a video stream including different video stream data for a plurality of respective layers, to a relay server, receiving, by the transmitting server, first section network state information which is information about a network state between the relay server and the transmitting server, from the relay server, receiving, by the transmitting server, second section network state information which is information about a network state between the plurality of respective user terminals and the transmitting server, and adjusting, by the transmitting server, qualities of video streams which are transmitted to the plurality of respective user terminals through the relay server by using network state information of the first section and network state information of the second section.