1. A source device for a communication system for transmitting video data including (a) three-dimensional video data including first video frame data and second video frame data and (b) two-dimensional video data including third video frame data from the source device to a sink device, the source device comprising: a first storage device for storing a first video format identification code table indicating a relationship among a video format information identifier, a type of video data, output specifications of the video data, and a format structure of the three-dimensional video data when the type of the video data is the three-dimensional video data, a first controller for, before transmitting video data to be transmitted, determining the video format information identifier of the video data to be transmitted by referring to the first video format identification code table based on the type of the video data to be transmitted, the output specifications of the video data to be transmitted, and the format structure of the three-dimensional video data when the type of the video data to be transmitted is the three-dimensional video data, for storing a determined video format information identifier into a video code field included in an output format notify message, and for transmitting the output format notify message to the sink device; and a transmitter for combining the first video frame data and the second video frame data into combined video frame data for every video frame when the video data to be transmitted is the three-dimensional video data, and for transmitting the combined video frame data to the sink device, the combined video frame data having the format structure included in the output format notify message.