PURPOSE: To prevent a winding from deformation or damage caused by the temperature change and improve dielectric strength by using insulation sheet whose thermal expansion coefficient has little difference from that of metal sheet with which the insulation sheet is laminated and wound.

CONSTITUTION: Metal sheet 22 and insulation sheet 23 are wound between cooling ducts 26 and as this insulation sheet 23 one of such types of films as polyamide film, polycrabonate film or fluorocarbon resin film is used to form a winding. When aluminum is used as the metal sheet 22 the difference of thermal expansion coefficient between the insulation sheet 23 and the metal sheet 22 is 2.7W6.7×10-5/°C which is 1/7.4W1/3 of the value of a conventional foil winding transformer which uses polyethylene. Thus, the difference of expansion between the insulation sheet 23 and the metal sheet 22 caused by temperature change is reduced and the damage of the surface of the metal sheet 22 or the insulation sheet 23 is prevented and breakdown between winding turns is avoided.