A constant voltage generating circuit according to the present invention includes a reference voltage generation source 1, a differential amplifier 2 that receives an output voltage (REF1) of the reference voltage generation source 1 at one terminal and receives a potential (VREF2), generated by adding a predetermined potential difference to a regulator voltage (VREG), at another terminal, and switching means 6 that controls the current amount of an output terminal 10 of the reference voltage generation source 1 so that the current amount increases for a fixed period of time immediately after the power is turned on. The switching means 6, which is turned on/off based on VREF2, stabilizes the output voltage (REF1) of the reference voltage generation source 1 quickly after the power is turned on and then stabilizes the regulator voltage (VREG).