The purpose of the present invention is 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. The invention relates to a flux layer 2 composed of solid constituents of 5-25 g/m2 formed on a surface of an aluminum plate member 1, by spray-coating a flux composition from a nozzle 20 to the plate member 1 with a thickness of 0.2-1.6 mm, which was press-molded and heated for degreasing in a heat-degreasing oven 10, while a temperature of the plate member 1 is kept from 120 to 180° C. under the remaining heat after completing a heating step for degreasing, and naturally drying for volatilizing volatile constituents of the flux composition utilizing the remaining heat.