Abstract: A regulator circuit is provided which suppresses a variation in output voltage upon occurrence of a variation in input voltage or output current without any increase in steady-state power consumption. The regulator circuit includes a detection circuit and an auxiliary circuit in addition to an error amplifier, an output transistor, a first resistor, a second resistor, and a reference voltage source. The error amplifier, the output transistor, the first resistor, and the second resistor form a typical linear regulator. The detection circuit includes a detecting capacitor, a first transistor, and a gain control resistor, which are serially connected between an input terminal and a ground terminal. A variation in input voltage leads to a change in potential at one end of the detecting capacitor with the input voltage. This causes a transient detecting current to flow, thereby allowing for detecting a voltage variation.