A reference voltage generator includes a mirroring circuit generating a first sub-voltage and a second sub-voltage that are constant, a first voltage generator including a first switch generating a first voltage based on the first sub-voltage, and a second voltage generator including a second switch generating a second voltage that is lower than the first voltage based on the second sub-voltage, wherein the second switch has a threshold voltage that is lower than the first switch to keep a voltage difference between the first voltage and the second voltage as a first reference voltage.