A method and apparatus for outputting a supplementary content in a WFD is disclosed. A method for outputting a supplementary content in a WFD may comprise the steps of: after a WFD source is connected to a WFD, transmitting, by the WFD source, an RTSP-based RTSP parameter request message to a WFD sink in order to perform a capability negotiation procedure with the WFD sink; transmitting, by the WFD source, an RTSP parameter response message to the WFD sink in response to the RTSP parameter request message; and after the capability negotiation procedure is performed, transmitting, by the WFD source, a transmission stream, obtained by overlapping and multiplexing a video stream including video data having first resolution and a supplementary stream including supplementary data having second resolution, to the WFD sink, wherein a WFD discovery response frame may include second resolution-related information for determining the second resolution and an RTSP parameter response frame may include the second resolution-related information.