A voltage detector for detecting whether an input voltage is no lower than a predetermined threshold voltage, comprising: a reference voltage generator configured to generate a reference voltage (Vref); and a comparator configured to receive the input voltage (Vin) 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, wherein the reference voltage generator includes: a first write MOS transistor (M1w) including a control gate and a floating gate; a second write MOS transistor (M2w) connected in series with the first write MOS transistor, the second write MOS transistor including a control gate and a floating gate; a first output MOS transistor (M1r) including a control gate and a floating gate that are electrically connected to the control gate and the floating gate of the first write MOS transistor (M1w); and a second output MOS transistor (M2r) connected in series with the first output MOS transistor, the second output MOS transistor including a control gate and a floating gate that are electrically connected to the control gate and the floating gate of the second write MOS transistor (M2w), the first write MOS transistor and the second write MOS transistor each include a tunnel oxide film that allows electric charges to tunnel therethrough to be injected into the floating gates, and the first output MOS transistor and the second output MOS transistor do not include the tunnel oxide film and the reference voltage is output from a connection point between the first output MOS transistor and the second output MOS transistor. a reference voltage generator configured to generate a reference voltage (Vref); and a comparator configured to receive the input voltage (Vin) 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, wherein the reference voltage generator includes: a first write MOS transistor (M1w) including a control gate and a floating gate; a second write MOS transistor (M2w) connected in series with the first write MOS transistor, the second write MOS transistor including a control gate and a floating gate; a first output MOS transistor (M1r) including a control gate and a floating gate that are electrically connected to the control gate and the floating gate of the first write MOS transistor (M1w); and a second output MOS transistor (M2r) connected in series with the first output MOS transistor, the second output MOS transistor including a control gate and a floating gate that are electrically connected to the control gate and the floating gate of the second write MOS transistor (M2w), the first write MOS transistor and the second write MOS transistor each include a tunnel oxide film that allows electric charges to tunnel therethrough to be injected into the floating gates, and the first output MOS transistor and the second output MOS transistor do not include the tunnel oxide film and the reference voltage is output from a connection point between the first output MOS transistor and the second output MOS transistor. a first write MOS transistor (M1w) including a control gate and a floating gate; a second write MOS transistor (M2w) connected in series with the first write MOS transistor, the second write MOS transistor including a control gate and a floating gate; a first output MOS transistor (M1r) including a control gate and a floating gate that are electrically connected to the control gate and the floating gate of the first write MOS transistor (M1w); and a second output MOS transistor (M2r) connected in series with the first output MOS transistor, the second output MOS transistor including a control gate and a floating gate that are electrically connected to the control gate and the floating gate of the second write MOS transistor (M2w), the first write MOS transistor and the second write MOS transistor each include a tunnel oxide film that allows electric charges to tunnel therethrough to be injected into the floating gates, and the first output MOS transistor and the second output MOS transistor do not include the tunnel oxide film and the reference voltage is output from a connection point between the first output MOS transistor and the second output MOS transistor.