To provide a flexible wiring board using a resin paste for a protective film of the flexible wiring board, which is excellent in low curvature property, flexibility, adhesiveness with a sealing material, solvent resistance and chemical resistance, thermal resistance, electrical property, humidity resistance, workability, and economic efficiency, which are required as the protective film of the flexible wiring board.

A method for forming the protective film of the flexible wiring board is provided with a step of printing the resin paste on an Sn-plated wiring pattern of the flexible wiring board, wherein the resin paste contains resin which is a thermosetting resin including a polyurethane structure expressed by a formula (1), inorganic particulates containing silica particulates, and a non-nitrogen system polar solvent, and a step of forming the protective film by heat-hardening the resin paste during heating at 80 to 130C.