PURPOSE: To increase a winding region and to equip with a more coil or winding, by causing transformer cores to form cut portions respectively and by fitting therein terminal pin holders of a bobbin.

CONSTITUTION: Cutting portions 1h, 2h are being formed at the core sides corresponding to faces 1f, 2f of transformer cores 1, 2. A bobbin 3 provides with brim portions 3b at both sides of a hollow portion 3a in which inner legs 1g, 2g can be inserted and terminal pin holders 3c each having an increased thickness below the brim portion 3b. First, a plural of coil 4 are wound on the bobbin 3 and each coil end is secured to respective terminal pins 6. Thereafter, the inner legs 1g, 2g of the transformer core 1, 2 are inserted from both the sides of the bobbin 3 respectively and the contacting faces of the cores 1, 2 are bonded together by an adhesive. In this way, the terminal pin holders 3c of the bobbin 3 can be fitted in the cut portions 1h, 2h of the transformer core 1, 2 so the terminal pin holders 3c and transformer cores 1, 2 may be bonded together by the adhesive respectively.