MPEG II system is disclosed, in which audio/video data multiplexed and transmitted in a form of program stream or transport stream depending on an ambient of application from a transmitter side is demultiplexed automatically according to the form of the stream at a receiver side for presenting the audio/video data, including a transport stream buffer for storing transport stream data synchronized to a first clock signal in an order of reception if a transport stream is selected from streams received in an ambient with many errors in response to a first control signal, a program stream FIFO buffer for storing program stream data synchronized to a second clock signal in an order of reception if a program data of a storage medium under an ambient without error is selected in response to a second control signal, a PES decoder for decoding elementary streams of audio and video to a PES level on the same time to provide status flags and packet data if the transport stream data or the program stream data provided from the transport stream FIFO buffer and the program FIFO buffer respectively are PES packet data, a PES buffer for storing a PES packet data decoded by the PES decoder, and an audio decoder and a video decoder for restoring the packet data from the PES buffer into original audio and video signals respectively.