To provide a constant voltage circuit capable of preventing reverse-current from a battery for backup and lowering the lowest operation voltage of a VR (voltage regulator).

Bias changeover switches 35, 37 for connecting a substrate with either of the side of a power supply 1 or the side of the battery 7 for backup are connected with the substrate of an output transistor 25. An output terminal of a comparator 39 for comparing input voltage Vbat with output voltage Vout is connected with the changeover switch 35 via the changeover switch 37 and an inverter 41, when the input voltage Vbat is higher than the output voltage Vout, the changeover switch 35 becomes on, the changeover switch 37 becomes off and when the input voltage Vbat is lower than the output voltage Vout, the changeover switch 35 becomes off and the changeover switch 37 becomes on.