In a switching DC/DC converter using an insulating transformer Tr, a primary coil and a secondary coil of the transformer Tr, an input choke coil L1, and an output choke coil L3 are wound around a common core and are formed into a single component. A primary circuit of the transformer Tr includes a switching device Q1 connected in series with the primary coil, an input choke coil L1 connected across the terminals of the primary coil, and a series circuit of a clamp capacitor C1 and a switching device Q1' which are connected across the terminals of the primary coil. A secondary circuit of the transformer Tr includes rectifying switching devices Q2 and Q2' connected to the secondary coil, the output choke coil L3, and an output smoothing capacitor C.sub.0. Further, a series circuit of the switching device Q1' and the clamp capacitor C1 may be connected across the terminals of the switching device Q1. The coils are arranged in directions of canceling DC fluxes generated by the coils, so that it is possible to greatly reduce the DC bias magnetization of the core, the volume of the core, and core losses, thereby achieving higher efficiency in the overall apparatus.