A home theater system comprising: an audio/video reproduction device; a display that displays a video; and an audio output control device connected to said audio/video reproduction device via a communication medium, wherein said audio/video reproduction device includes an audio data acquisitor (110; 500) that acquires audio data, a sound volume adjustment operation information acquisitor that acquires sound volume, adjustment operation information representing a sound volume, adjustment operation of said audio output control device, a first transmitter/receiver that transmits said audio data acquired by said audio data acquisitor to said audio output control device via said communication medium, and transmits and receives a command to and from said audio output control device via said communication medium, characterized in that the audio/video reproduction device further comprises a storage that stores sound volume control status data relating to sound volume control of said audio output control device, and a first controller that controls said display and said first transmitter/receiver, said audio/video reproduction device is configured to be settable to a theater mode for transmitting the audio data to said audio output control device, said first controller transmits a command for requesting the sound volume control status data to said audio output control device via said first transmitter/receiver when said audio/video reproduction device is set to the theater mode, and said sound volume control status data includes maximum sound volume value data representing a maximum value in a sound volume adjustable range of said audio output control device, and sound volume setting value data representing a sound volume of said audio output control device, said audio output control device includes a second transmitter/receiver that receives audio data from said audio/video reproduction device via said communication medium, and transmits and receives a command to and from said audio/video reproduction device via said communication medium, an amplifier that outputs an audio signal based on the audio data received by said second transmitter/receiver, and a second controller that controls said second transmitter/receiver and said amplifier and transmits current sound volume control status data relating to said audio output control device to said audio/video reproduction device via said second transmitter/receiver when receiving the command for requesting the sound volume control status data from said audio/video reproduction device via said second transmitter/receiver, said first controller updates, when the sound volume control status data has been received from said audio output control device via said first transmitter/receiver after transmitting the command for requesting the sound volume control status data, the sound volume control status data stored in said storage with the received sound volume control status data, determines, when sound volume operation information has been acquired by said sound volume operation information acquisitor while said audio/video reproduction device is set to the theater mode, sound volume setting value data after a sound volume adjustment operation based on the acquired sound volume adjustment operation information and the sound volume control status data stored in said storage, and transmits a command including the determined sound volume setting value data to said audio output control device via said first transmitter/receiver while displaying a ratio of the current sound volume of said audio output control device to a maximum value in the sound volume adjustable range of said audio output control device based on said determined sound volume setting value data and the maximum sound volume value data stored in said storage, and said second controller controls, when the command including the sound volume setting value data has been received from said audio/video reproduction device via said second transmitter/receiver, a sound volume of said amplifier based on the received sound volume setting value data. an audio/video reproduction device; a display that displays a video; and an audio output control device connected to said audio/video reproduction device via a communication medium,