PURPOSE: To obtain a fly-back transformer for TV receiver having excellent dielectric property and of which hardened resin structure is improved in crack resistibility by impregnating the transformer with a combined resin composed of a flexible polyol and isocyanate having proper reactivity.

CONSTITUTION: Polyol A consists of at least one kind of liquid polybutadiene homopolymer of which molecular weight is 500W10,000 and having hydroxyl groups at both ends of molecule, liquid stylene butadienecopolymer of which molecular weight is similar and having hydroxyl groups at both ends of molecule and liquid acrylonitrile butadienecopolymer of which molecular weight is similar and having hydroxyl groups at both ends of molecule is mixed with polyol B consists of at least one kind of telomerized diol and telomemerized triol. At this time, assuming A+B=100, weight ratio of A and B is set to (70W97):(30W3), and this is added with a specified amount of polyisocyanate shown by the formula, hydrated alumina powder and red phsphorus powder respectively.