The present inventio relates to a calibration circuit. Impedance adjusting transistors are once inactivated on every occasion of changing an impedance adjusting code. After restoring the potential toan initially set potential by once inactivating the impedance adjusting transistors, the state of the transistors is switched according to the impedance adjusting code. By starting the potential fromthe initially set potential at the time of switching the state of the transistors, no switching noise is generated. Since no switching noise is generated, a comparator always carries out stable comparison and judgment and ensures stable outputs.