An insulation type step-down converter includes first and second step-down transformers each of which includes an input-side coil and an output-side coil. An intermediate portion of the output-side coil of the first step-down transformer and an intermediate portion of the output-side coil of the second step-down transformer are connected to each other. First, second, third, and fourth rectifier elements are connected in series with first, second, third, and fourth output-side coils, respectively. Smoothing coils are connected to the first to fourth output-side coils. The first, second, third, and fourth rectifier elements are connected such that electric currents flow simultaneously in the first output-side coil and the third output-side coil, and electric currents flow simultaneously in the second output-side coil and the fourth output-side coil in a manner alternating with the electric currents in the first output-side coil and the third output-side coil.