To suppress the underflow of a P-STD audio buffer in real-time even when the length of one pack to be transmitted is large in the case of PS multiplexing.

A multiplexer 15 is provided with a reproduction audio ES amount calculating part 155 and a buffer occupancy amount at a decoder side is virtually calculated. An output PS is selected by the occupancy amount and an encoder side occupancy amount. When a pack only for a video is outputted, it is judged whether the underflow occurs in the P-STD audio buffer 154A during the output or not and the PS output is stopped till a state for outputting an audio stream is obtained in the case of the occurrence of the underflow. When an output possible state is obtained, the pack comprising audio is outputted so that the occurrence of the underflow is suppressed.