The invention relates to a flux layer (2) composed of a solid component of 5 to 25 g/m<2>is formed on the surface of the sheet member (1), the method comprises that a sheet member (1) made of aluminum with a sheet thickness of 0.2 to 1.6 mm, at the time when the temperature of the sheet member (1) is 120 to 180DEG C by remaining heat after the completion of the heating-degreasing stage, the spray coating of a flux composition is performed from a nozzle (20), utilizing the remaining heat, naturally drying, then the volatile component in the flux composition is volatilized, wherein the aluminum sheet is subjected to press working and heated and degreased in a heating-degreasing furnace (10). The purpose of the present invention is to provide a method of forming a flux layer on an aluminum plate member, wherein a heating step for drying after a spray-coating of a flux composition can be omitted.