According to deployment of broadband access networks like FTTH (Fiber-To-The-Home), many Internet Service Providers in Japan start providing video broadcasting services via IP multicast. As the service develops into higher quality and more diverse, a broadcasting system capable of handling high bit-rate contents will be required. However, typical implementation of broadcasting server may have problems because of poor packet timing control precision. In this paper, we present a new transmission control method for IP multicast that is synchronized with TCP flow control, which can be applied to QoS-guaranteed networks. We evaluate its validity using an experimental system with HDTV (High Definition TV) streams. We are able to demonstrate that the system can simultaneously distribute eight-channel HDTV streams.