PURPOSE: To improve the efficiency of a transformer and to facilitate the mounting of a coil by axially splitting a bobbin, rotatably mounting it on legs of a core having a magnetic path including no disconnection, and externally driving to wind the coil.

CONSTITUTION: A core 40 is composed of a box-shaped portion 41 and a columnar leg 42 integrally formed from the portion 41 at the center of the portion 41. A bottom 50 is split into two sections 51 and 52. After the two sections 51, 52 are opposed, they are rotatably mounted on the leg 42. The end of the coil is mounted on the terminal 56 of the bobbin. A drive gear coupled with a motor is engaged with a gear 55 formed at the collar 53 of the bobbin 50 to rotate the bobbin 50, thereby winding the coil on the bobbin 50.