A conventional power supply device has a problem in miniaturization. A power supply device generates a prediction value of an error signal from first and second error signals, and controls an output voltage so that the prediction value lies between first and second threshold values. The first error signal is obtained by converting an error voltage based on the difference between the output voltage and a reference voltage at a first timing. The second error signal is obtained by converting an error voltage based on the difference between the output voltage and the reference voltage at a second timing.