To provide a Pb-free material having the excellent corrosion resistance and a method of coating a steel plate by forming at least a surface of a steel tank for contact with the fuel with a non-electrolytic Ni-P group alloy film.

This tank has an opening at a part obtained by welding after pressing a cold drawing steel plate, and at least a surface thereof for abutting on the fuel is formed with a Ni-P group alloy film by non-electrolytic plating. An acid side non-electrolytic plating liquid is desirably used because of the high metal depositing speed thereof. As this acid non-electrolytic plating liquid, a liquid material mainly composed of at least one kind of nickel salt such as nickel sulfate and nickel chloride, and the reduction agent of hypophosphite, and at least one kind of the complexing agent to be selected among succinic acid, malic acid and sodium salt thereof. With this composition, the fuel tank can be coated with the Pb-free Ni-P group alloy film having the excellent corrosion resistance without generating the earth environmental problem.