A transformer that improves the DC superposition characteristic without incurring eddy-current losses. In the transformer, a part of a plate-like core opposing a top face of a flange of a drum core is formed with a first opposing portion opposing none of input and output terminals and a second opposing portion opposing the input and output terminals. A first gap is formed between the top face and the first opposing portion by a spacer. A second gap greater than the first gap is formed by a recess of the plate-like core provided so as to correspond to the second opposing portion. This allows magnetic fluxes to pass between the top face and the first opposing portion where the gap is formed and inhibits them from passing between the plate-like core and the input and output terminals where the second gap greater than the first gap is formed.