A data multiplexing apparatus or the like is suitable for use with a digital satellite broadcasting system or the like. Inputted video data PESa may be converted by a P/S converter (142) in the form of parallel data of byte unit to serial data and then may be written in a buffer (143) and stored therein. Data stored in the buffer may be read out under control of a multiplexing control section (135). The data thus read out may be converted by a S/P converter (144) in the form of serial data to parallel data of byte unit and thereby generated as outputted data PDa. When the data storage quantity of buffer increases, under control of a rate-variable control section (147), a data quantity may be reduced by discarding DCT coefficients of high-orders, for example, with reference to analyzed result of a data analyzing section (146) and thereby the rate may be varied. Thus, the increase of the data storage quantity of buffer can be suppressed and the increase of the delay time caused when inputted data are multiplexed can be avoided. Therefore, there can be prevented a disadvantage such as a failure of synchronization on the receiving side.