To widen the range of color selection and at the same time, to form a beautifully coated face by applying a second synthetic resin powder coating material while waste heat is retained after thermally melting a first synthetic resin powder coating material onto a metal product.

When applying a synthetic resin powder coating material to a metal surface, first a first synthetic resin powder coating material is thermally melted onto the metal surface and then, a second synthetic resin powder coating material is applied to the metal surface while the heat of the metal surface is retained. In this case, the mixture ratio o f the second synthetic resin powder coating material for the first synthetic resin powder coating material is 3-15wt.%. In addition, the surface temperature of the metal surface when the second synthetic resin powder coating material is applied is set to 250°C or less. Further, when forming a double coating film as described, a metal product heated to about 300°C is placed in a fluid dipping bath in which the first synthetic resin powder coating material is afloat, then is placed in another fluid dipping bath in which the second synthetic resin powder coating material is afloat, and finally is taken out of the fluid dipping bath to be cooled.