Primary and secondary coils are provided in the first through section and a coil group is also provided in the second through section. Hence, the surface area over which the coil group extends within a plane which is perpendicular to the through sections is greater than in the case where all of the coils are provided in a single through section. The surface area which is not covered by the magnetic body cores of the platelike members increases. In cases where the surface area of the members is large, the heat radiation characteristic is enhanced. Hence, the cooling efficiency of the transformer improves. In cases where there is a plurality of coil groups which are magnetically coupled to one another in particular, because it is difficult to move the heat produced in the plurality of coil groups through heat conduction, heat transfer, or heat radiation, a heat radiation structure of this kind is effective.