PURPOSE: To form an FSK demodulating circuit of different specifications by switching clocks, by using a switched capacitor filter (SCF) as a pre-band pass filter.

CONSTITUTION: A receiving signal is inputted to an SCR19 through a microphone 15, a high pass filter consisting of a capacitor 16 and a resistance 17, and an amplifier 18. A clock frequency is super-posed onto the output of the SCF in steps, and therefore, the offset due to the operational amplifier of the SCF is eliminated by a buffer 22, a capacitor 23, and a resistance 24 after the output passes through a low pass filter. Next, it is discriminated whether the signal is a mark or a space by an amplifier 25, a limiter 26, a comparator 27, and a demodulating circuit 28 to obtain a digital signal. The clock of the SCF is obtained by an oscillating circuit 30 having two frequency division ratios and a quartz oscillator 29, and a frequency dividing frequency adapted for a high group or a low group is given in accordance with a high-band receiving or a low-band receiving by the input of H or L in respect to the frequency division ratio.