A DC—DC converter that receives input of a direct current voltage such as a battery and supplies a controlled direct current voltage to a load is simplified. It comprises a voltage step-down converter section consisting of a first switch 2, a first rectifying means 3 and an inductor 4, a voltage step-up converter section consisting of an inductor 4, a second switch 5 and a second rectifying means 6, and a control section consisting of an output capacitor 7, an error amplifying circuit 10, an oscillation circuit 11 and a pulse width control circuit 12. An oscillation voltage Vt from the oscillation circuit 11 is compared with an error voltage Ve from the error amplifying circuit 10 and a duty ratio of each switch is adjusted to control the operation of voltage step-down, voltage step-up and step-down, and voltage step-up.