PURPOSE: To obtain an LPF following surely a change in a sampling frequency with simple circuit constitution by setting a switching frequency to drive a switched capacitor filter to a constant multiple of a PCM sampling frequency.

CONSTITUTION: A PCM modulation circuit consists of a switched capacitor (SCP)1 constituted as an LPF, an A/D converter 2 and a frequency divider 3. Through the constitution above, the cut-off frequency of the SCP1 is set to fs/2 or below with respect to the sampling frequency fs. In varying the sampling frequency in the PCM modulating circuit, a clock signal fc synchronizing therewith is varied. Then a PCM demodulating circuit consists of a D/A converter 7, an SCP 8 and a frequency divider 9. Through such constitution as above, when the sampling frequency fs is changed, the cut-off frequency of the LPF is varied in following to the change in the sampling frequency.