PURPOSE: To provide the constant voltage circuit which performs switching operation at a high speed and is easily varied in set voltage.

CONSTITUTION: This circuit is equipped with means MP1, MN2, AD1, G3-G5, and 100 which output an output potential VOUT determined by the mid-point potential between resistances R1 and R2 connected in series between a power source potential VDD and a ground potential GND and make the resistance value of the resistance R1 or R2 apparently small when an input voltage varies. Consequently, the reaction time of variation in output potential responding to variation in the input potential is shortened. The output potential is optionally set with the ratio of the resistance values of the resistances R1 and R2.