PURPOSE: To contract the size of the externl form of the composite transformer by making the leg width of a central leg larger than the leg width of left and right legs and shaping a yoke section connecting the left and right legs and the central leg in thin tabular form.

CONSTITUTION: The leg width of the central leg 5S of the core 5 is set in approximately twice as large as the leg width of the left and right legs 5L, 5R. The yoke section 5M connecting the central leg 5S and the legs 5L, 5R is shaped tabularly, and the plate thickness is formed considerably thinly. When the composite transformer is formed as shown in the figure by using the core 6 with the same shape as the core 5, the height 2h of the transformer is reduced because the yoke sections 5M, 6M are molded thinly. Length 2l between the outer surfaces of coils 7, 8 in the direction of the left and right legs is shortened because the thickness of the central legs 5S, 6S is decreased. Accordingly, the external form of the composite transformer is contracted because the height 2h and length 2l of the transformer are shortened.