A supply voltage sensing circuit comprises an internal power supply circuit, which provides a constant output voltage regardless of the supply voltage. A delay circuit generates a delayed signal by delaying a variation in the output voltage. A divider circuit generates a divided voltage by dividing the supply voltage at a certain division ratio. A p-type MOS transistor has a source given the delayed signal and a gate given the divided voltage and turns on when the supply voltage lowers below a certain value. An output circuit provides an output voltage based on a drain voltage on the p-type MOS transistor.