Simplification of the method for controlling a step-up/step-down DC-DC converter to which the DC voltage of a battery or the like is inputted and which supplies a controlled DC voltage to a load. There are provided a step-down converter part comprising a first switch (2), a first rectifying means (3) and an inductor (4); a step-up converter part comprising the inductor (4), a second switch (5) and a second rectifying means (6); an output capacitor (7); and a control part comprising an error amplifying circuit (10), an oscillating circuit (11) and a pulse width controlling circuit (12). The oscillation voltage (Vt) from the oscillating circuit (11) is compared with the error voltage (Ve) from the error amplifying circuit (10) by use of an arrangement that in the step-up operation mode, the smaller the input/output voltage ratio is, or in the step-down operation mode, the greater the input/output voltage ratio is, the more the ratio of the oscillation voltages (Vt) during increasing or decreasing thereof varies. Then, the time ratios of the switches are adjusted to control the step-down, step-up/step-down and step-up operations.