An FSK signal receiver includes an amplifier which amplifies a first FSK signal at an adjustable gain. A frequency converter converts an output signal from the amplifier into a second FSK signal having a frequency lower than a frequency of the first FSK signal. A demodulator demodulates the second FSK signal into a baseband signal. A bit-state detector detects a bit state from the baseband signal in response to a threshold. A calculator calculates a number of times of occurrence of a fact that the baseband signal moves across the threshold during a given time. The gain of the amplifier is adjusted in response to the number of times which is calculated by the calculator.