A VLD decodes MPEG2 data so as to generate RUN and Level therefrom. A multiplication unit multiplies target data to be added to the Level, and sends a multiplied result to an adder. The adder adds the target data to the Level. The VLC codes the MPEG2 data, using the RUN and the Level to which the target data is added. The selector compares the data length of the MPEG2 data which has been decoded by the VLC with the data length of the original MPEG2 data. If those data lengths coincide with each other, the selector selects the MPEG2 data output by the VLC. On the contrary, if those data lengths do not coincide with each other, the selector selects and outputs the original MPEG2 data. A counter counts the number of times the selector has done the selection, while another counts the number of times the selector has selected the data output by the VLC. The comparator compares the counted values of both of the counters, and adjusts the insertion degree based on a result of the comparison.