An insulation type step-down converter includes first, second, third, and fourth secondary-side coils, and first, second, third, and fourth rectifier elements. The first, second, third, and fourth rectifier elements is capable of performing rectification such that electric currents flow alternately only in one of the first and second secondary-side coils and one of the third and fourth secondary-side coils, and electric currents flowing simultaneously in one of the first and second secondary-side coils and one of the third and fourth secondary-side coils are opposite in direction to each other so as to cancel out a magnetic flux passing through the middle leg each time when electric current flowing in the primary-side coil is changed in direction. Provided is an insulation type step-down converter which can minimize an increase in heat generated by the primary-side coil even at a large step-down ratio of a step-down transformer without raising manufacturing costs.