PURPOSE: To prevent insulation reduction due to deformation of a foil coil by heat-treating the insulation sheet of a foil coil by means of hot air or a hot roll above the maximum operating temperature which a metal sheet experiences on operation and then using an insulation sheet without any heat shrinkage problem.

CONSTITUTION: A foil coil is constituted of one metal sheet 2 and two insulation sheets 3. The insulation sheet 3 is subjected to heat treatment by means of hot air or a hot roll above the maximum operating temperature which the metal sheet 2 experiences on operation to eliminate thermal shrinkage, thus reducing such deformation as wrinkle and warpage of an edge part when manufacturing the coil winding and obtaining a foil winding transformer which is reliable for a long time without reducing insulation performance.