PURPOSE: To increase a threshold limit value for a generated voltage largely by providing diodes on step-like stairs and securing the distance between the diodes and a core to keep the creeping distance.

CONSTITUTION: A coil bobbin 8 has collars 10 and 11 on which a plurality of terminal pins 9 are planted. The bobbin is wound with high voltage windings 13aW13c divided into a plurality of pieces on a low voltage winding. A step-like stair 14 is formed on the outer side of the collar 10. A core 15 of the EI-type and EE-type made of ferrite and so forth is incorporated in the bobbin 8. A plurality of diodes 16aW16d are incorporated in the step-like stair 14 of the bobbin 8. The diode 16d which outputs the lowest voltage output is arranged at the lowest stair. As the voltage output from the diode become higher, the diodes 16c, 16b and 16a are arranged at the sequentially higher stairs 14c, 14b and 14a. The diodes 16aW16d are connected with the terminal pins 9 provided on the collar 10 and electrically connected with the high voltage windings 13aW13c respectively.