A transformer has a bobbin made of a resin material having flanges (2a to 2f) formed thereon, at least three chambers defined between the adjacent flanges. A primary auxiliary winding (P2), which supplies an electric power to an drive-control and overvoltage protection circuit and a primary excitation winding (P1-1) are wound on the chambers (3a and 3c) with at least one chamber (3b) disposed therebetween. A secondary winding (S2) which consumes the electric power always is wound on the chamber (3b) between the chambers (3c and 3a) on which the primary excitation winding (P1-1) and the primary auxiliary winding (P2) are wound.