Abstract: An A/D converter (1) converts analog modulation signals, such as video signals and audio signals, into digital modulation signals. A center frequency data setting circuit (6) outputs center frequency data for setting the center frequency of an FM wave. An operation unit (5) outputs such addition data, on the basis of the center frequency data and average frequency data from a counter circuit (4) as will make the average frequency of the FM wave substantially identical with the center frequency that has been set. An adder (2) adds the digital modulation signals and the addition data, and outputs addition digital signals. A DDS (3) accumulates an addition digital signal in every sampling period supplied from a reference oscillator (7). In the ROM (32) of the DDS (3) are stored sine wave data and the ROM (32), having the accumulated signals as its address input, outputs the sine wave data.