A producing method of a flexible wired circuit board that can prevent the formation of a gap between an elongate substrate and a stiffener sheet bonded thereto to prevent contamination of the flexible wired circuit board obtained. In the process subsequent to the process of forming a conductive pattern 3 on a surface of the elongate substrate 1 by the semi-additive process using electrolysis plating and then annealing the elongate substrate 1 with the conductive pattern 3 in its wound up state, a stiffener sheet 9 having a width narrower than the elongate substrate 1 is bonded to the back side of the elongate substrate 1. Thereafter, an oxidized film formed on a surface of the conductive pattern 3 is removed and then a solder resist 11 is formed thereon. This prevents the strip of the stiffener sheet 9 from the elongate substrate 1 and in turn prevents etching solution or developing solution from entraining in a gap therebetween.