A semiconductor integrated circuit device for a magnetic disk apparatus has analog circuits such as a read/write circuit and digital circuits such as an interface driver circuit, a control circuit, and a stepping motor driver circuit, all of these circuits operating on a single supply voltage. The semiconductor integrated circuit device further has a voltage regulator whose output voltage is lower than the supply voltage and variable according to the voltage applied to an output voltage adjustment terminal. The control circuit operates on the output voltage of this regulator.