A constant-voltage power source circuit includes a first current mirror circuit having a diode and a first transistor and a second current mirror circuit having at least second and third transistors, which sets a current flowing to the first current mirror circuit to be substantially equal to a current flowing to the second mirror circuit, and provides the current flowing to the second current mirror circuit to a fourth transistor that determines an output voltage. With this arrangement, a current flowing to the current mirror circuits structured by the diode and transistors is determined by a forward characteristic of the diode, and this current becomes a constant current which is hardly affected by a variation of a power source voltage. When this current flows to the two current mirror circuits including transistors and then flows to the transistor which determines an output voltage, it is possible to hold a voltage between the base and the emitter of the transistor which determines an output voltage, at substantially a constant value without being affected by a variation of the power source voltage.