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. First, second, third, and fourth rectifier elements are connected in series with first, second, third, and fourth series coils, respectively, the first, second, third, and fourth series coils each having the output-side coil of the first step-down transformer and the output-side coil of the second step-down transformer connected in series. The first to fourth series coils are connected to smoothing coils. The connection is such that electric currents flow simultaneously only in one of the first and second series coils and one of the third and fourth series coils in an alternate manner, and electric currents flowing simultaneously in one of the first and second series coils and one of the third and fourth series coils are opposite in direction to each other.