In order to provide a semiconductor IC unit such as a microprocessor, etc., which satisfies both fast operation and lower power consumption properties with its high quality kept, the semiconductor IC unit of the present invention is composed so as to include a main circuit (LOG) provided with transistors, which is formed on a semiconductor substrate, and a substrate bias controlling circuit (VBC) used for controlling a voltage to be applied to the substrate, and the main circuit includes switching transistors (MN1 and MP1) used for controlling a voltage to be applied to the substrate and control signals output from the substrate bias controlling circuit is entered to the gate of each of the switching transistors and the control signal is returned to the substrate bias controlling circuit.