The present invention provides a regulator circuit that can fast-respond to a variation in load current and supply a sufficient drive current so as to be capable of generating a stable internal source voltage. The regulator circuit includes a preamplifier circuit that detects and amplifies a different between a reference voltage and an internal source voltage, a clamp circuit that limits the amplitude of an output of the preamplifier circuit, a main amplifier circuit that amplifies the amplitude-limited output of the preamplifier circuit, and a driver circuit that outputs the internal source voltage according to the output of the main amplifier. Even though the internal source voltage varies abruptly, the regulator circuit does not oscillate owing to the effect of the clamp circuit.