To obtain a constant-voltage power supply circuit whose power consumption is reduced and whose output voltage stability is improved.

When an external power supply 1 is closed, an input voltage, inputted through an input terminal P1 starts rising, a starting circuit 6 is operated, and an output voltage control transistor Tr1 is operated through a drive circuit 2 to elevate an output voltage Vout. When the output voltage Vout is elevated sufficiently, the starting circuit 6 discontinues the starting operation, a differential amplifier 5 amplifies the differential voltage between a reference voltage which is outputted from a reference voltage-generating circuit 4 and the monitored voltage of the output voltage Vout, which is outputted from an output voltage monitoring circuit 3 and outputs the amplified differential voltage as a control voltage and controls the transistor Tr1 through the drive circuit 2 to maintain the output voltage at a constant value. The reference voltage generating circuit 4 and the differential amplifier 5 are provided between an output terminal P2 and a ground and can be driven by the output voltage Vout, which has a potential lower than the input voltage and is maintained at the constant value.