PURPOSE: To attain simple circuit constitution by constituting a frequency discriminating circuit executing FSK demodulation with a tuning circuit where a frequency having almost no tuning frequency and tuning gain is slightly higher or lower than the frequency obtained by applying FSK modulation to a digital signal.

CONSTITUTION: The frequency f4 having almost no tuning frequency f3 and tuning gain of a tuning circuit comprising a coil L3 and a capacitor C3 is set slightly higher or lower than the frequencies f5, f6, of the signal obtained by applying FSK modulation to the digital signal. The signal having the frequencies f5, f6 obtained by applying FSK modulation to 1, 0 of the digital signal inputted to a transistor TR3 is inputted to the said tuning circuit, its output is rectified by a diode D3 and demodulated to a resistor R8 via a resistor R7 as a signal B.