A negative voltage generator includes a direct current voltage generator configured to generate a direct current voltage, a reference voltage generator configured to generate a reference voltage, an oscillator configured to generate an oscillation clock, a charge pump configured to generate a negative voltage in response to a pump clock, and a voltage detector. The voltage detector is configured to detect the negative voltage by comparing a division voltage, obtained by voltage dividing the direct current voltage, with the reference voltage, and to generate the pump clock corresponding to the detected negative voltage based on the oscillation clock.