An insulating varnish forms an insulating coating film having a shape that corresponds to the shape of an opening of a die, having a uniform thickness. The insulating varnish is applied onto a surface of a conductor, subsequently passes through a die to remove the excess applied insulating varnish, and is then dried or baked to form an insulating coating film on the surface of the conductor. The insulating varnish has a viscosity of 10 Pa·s or more measured by a B-type viscometer at 30° C. The insulating varnish preferably contains no filler, and is preferably a polyimide precursor solution. Since the insulating varnish has a high viscosity, baking and solidification can be performed while maintaining a shape formed when the insulating varnish passes through a die.