In order to increase the transient response speed of a DC-to-DC converter capable of voltage step-up and voltage step-down, to which a DC voltage is input from a battery or the like and from which a controlled DC voltage is supplied to a load, a first voltage E1 lower than an output setting voltage Eset by a predetermined voltage is defined and is compared with a DC output voltage Eo by a first comparator, and the result of the comparison is output. @a A second voltage E2 lower than a DC input voltage Ei by a predetermined voltage is defined and is compared with the DC output voltage Eo by a second comparator 133, and the result of the comparison is also output. When the results of the comparisons are represented by Eo