An object of the invention is to provide a micro power converter of a step-up and step-down type without requiring more than two semiconductor switches, without increasing the size of a semiconductor chip, and without degrading efficiency. A micro power converter comprises a micro transformer composed of a planar transformer having a structure including a conductor wound on and through a planar magnetic core, and a semiconductor chip including semiconductor switches S1, S2, and a control circuit for controlling the switches. By constructing a flyback transformer using a micro transformer composed of a planar transformer, a micro power converter of a step-up and step-down type is provided having two semiconductor switches and an overall size comparable to a conventional micro power converter.