An FDM-TDM transmultiplexing system for a modulation/demodulation device which is applicable to a regenerative repeating system of a satellite or a ground radio communication system is disclosed which uses chirp-z-transform. A chirp filter is implemented with a digital circuit. The circuit scale of the digital chirp filter increases in proportion to a square root of the total number of channels N, enhancing miniaturization of an FDM-TDM transmultiplexer.