A step-down circuit for power supply voltage, which converts an external power supply voltage into a first voltage that is lower than the external power supply voltage so as to apply it to an internal circuit, is provided with: a reference voltage generation circuit for generating a reference voltage from an external power supply voltage, a differential amplification circuit for releasing a difference between the reference voltage and the first voltage as a control signal, a driving circuit for controlling a driving current to be supplied to the internal circuit according to the control signal from the differential amplification circuit, a signal generation circuit for releasing a detection signal by detecting an increase of consumption current in the internal circuit, and a control means for controlling the driving circuit so as to increase the current to be supplied to the internal circuit in accordance with the detection signal from the signal generation circuit. Even if the consumption current of the internal circuit increases, this arrangement makes it possible to restrict the drop of the first voltage to be applied to the internal circuit to a minimum value.