A frequency-shift-keying demodulator including a phase shifter for shifting the phase of an input signal by a predetermined degree and outputting a shifted signal, a combining unit for combining the input signal and the shifted signal and for outputting a corresponding signal, and a low-pass filter for filtering the signal output by the combining unit and outputting a low-pass filtered signal, the bandwidth of the low-pass filter being matched with the bandwidth of a data signal included in the input signal, such that the combining unit includes at least one adder for adding the input signal and the shifted signal, and outputting an added signal, and at least one square law detector for receiving the added signal output by the adder and outputting a squared signal which is the square of the added signal, wherein the combining unit outputs the squared signal to the low-pass filter.