Method of manufacturing a multilayer flexible printed circuit board

A method of manufacturing a multilayer flexible printed circuit board having a strip line foldable at a folding part, the method comprising:

• a process of preparing a laminate body having a flexible insulative substrate, an inner layer circuit pattern provided inside the flexible insulative substrate and including a signal line extending in a predetermined direction, and metal foils covering both sides of the flexible insulative substrate;
• an opening part forming process of removing the metal foils at least in the folding part out of a ground layer formation scheduled region to form an opening part from a bottom face of which the flexible insulative substrate is exposed;
• a nonelectrolytic plating process of forming a nonelectrolytic plating coat on the flexible insulative substrate that is exposed from the bottom face of the opening part by a nonelectrolytic plating method;
• an etching mask forming process of forming an etching mask covering the nonelectrolytic plating coat;
• an exposed part forming process of removing the nonelectrolytic plating coat that is not covered by the etching mask to form a ground thin film constituted of the nonelectrolytic plating coat and an exposed part from which the flexible insulative substrate is exposed;
• and
• a protective layer forming process of forming a protective layer that covers the ground thin film and is in close contact with the exposed part.