To provide an audio-video data synchronization method whereby an increase in the amount of control message transmission can be suppressed and the video output can be prevented from being interrupted. The audio-video data synchronization method includes: obtaining, by the audio output device, a first latency that is a period of time taken for the video output device to output the video, by receiving the display identification data that includes information indicating the first latency from the video output device when the indication of the connection validity status information changes from Invalid to Valid; adjusting, by the audio output device, timing of outputting the audio according to the first latency; obtaining, by the video output device, a second latency which is a period of time taken to output the video in the outputting; notifying the audio output device of the second latency by sending a control message from the video output device when the connection validity status information indicates Valid, the control message including information that indicates the second latency obtained in the obtaining of a second latency; and adjusting, by the audio output device, timing of outputting the audio according to the second latency.