A voltage detector for detecting whether an input voltage is no lower than a predetermined threshold voltage, includes a reference voltage generator configured to generate a reference voltage, and a comparator configured to receive the input voltage and the reference voltage and to detect whether the input voltage is no lower than the threshold voltage that is determined by the reference voltage. Here, the reference voltage generator includes a first write MOS transistor, a second write MOS transistor, a first output MOS transistor and a second output MOS transistor each including a control gate and a floating gate.