A manufacturing method uses a solid coating die for applying an insulation varnish around a flat wire conductor. The die includes a die body and a die hole formed through the die body, into which the conductor is inserted. The die hole includes an entry portion having a cross section monotonically decreasing along a conductor insertion direction, and a coating portion including at least a sub-portion having a constant cross section. The cross section of the coating portion is a rectangle having four straight sides and four rounded corners. Each inner surface of each straight side has an inwardly projecting protrusion, a top contour of each protrusion being a circular arc, an elongated circular arc, an elliptical arc or a combination of these arcs, the maximum curvature of the top contour of each protrusion being larger than the maximum curvature of inner surface of the rounded corners.