A system and method are disclosed for providing a FSK demodulator using DLL and a demodulating method which detects a time order of the rising edges of square waves that correspond to two modulation frequencies and an in-between frequency and demodulates the relevant frequencies into data. The FSK demodulator includes a band-pass filter, an amplitude limiter for converting a waveform of the frequency filtered into a square wave, a delay line for receiving the square wave from the amplitude limiter and delaying the square wave for a delay time, a delayed flip-flop (DFF) for receiving an output signal from the amplitude limiter and an output signal from the delay line, determining which rising edge of the two input signals is earlier at a given time, and outputting the result of the determination as data, and a DLL circuit that locks the delay time of the delay line.