An FSK shift keying device includes a detector for detecting a transmission frame consisting of a preamble, data, and a postamble, a time constant circuit charged in response to a voltage signal detected by the detector for generating a time constant, a charging voltage being determined in accordance with the time constant set in correspondence with a duration of high level in the preamble of the transmission frame, a reference voltage generator for generating a reference voltage signal having an amplitude value which is 1/2 an amplitude value of the voltage signal from the detector, and further generating a comparison reference voltage signal obtained by subtracting the reference voltage from the charging voltage, and a comparator for comparing the comparison reference voltage signal with the voltage signal input from the detector.