Abstract: It is to establish a solder precoated conductor circuit substrate capable of conducting the fine pitch mounting and being excellent in the productivity and a technique of producing the same. In the solder precoated conductor circuit substrate, the solder layer formed on the conductor for the connection of electronic component is constituted with Sn thin film layer formed by Cu-Sn substitution reaction based on, for example, the Cu complex formation of thiourea and a Pb-coated Sn layer formed by covering at least a part of Sn crystal grains formed through Sn unhomogeneous reaction based on selective precipitation on Sn with Pb through Sn-Pb substitution reaction based on ionization tendency, in which the solder layer is desirably heated and melted and thereafter cooled to form an alloy layer.