In an FSK signal detector, an output from a limiter amplifier and an FSK signal are supplied to a comparator. The comparator has its output connected to two channels each composed of a correlator, an absolute value calculator and a digital low-pass filter in this order. The correlators calculate correlation between the output of the comparator and correlation signal strings (Cn to C2n, Dn to D2n) exhibiting certain periodicity. Outputs from the digital low-pass filters are calculated by a subtractor to be developed as demodulated data from a sign determination circuit. An FSK signal detector is provided which is capable of eliminating the adverse effect of fluctuations of an FSK signal.