To provide a method for forming a rustpreventive film on an ALC reinforcing strip where the film thickness of a rustpreventive film can be controlled, and the film thickness can be reduced without deteriorating rustpreventive performance.

When a rustpreventive film is formed on an ALC reinforcing strip, the reinforcing strip is dip-coated with a rustpreventive liquid, and is dried at 40 to 60C, thus a rustpreventive film of the first layer with a film thickness of 50 to 100 m is formed. Next, water is sprayed on the surface of the rustpreventive film of the first layer, and thereafter, the surface is dip-coated with a rustpreventive liquid, and is dried at 70 to 90C, thus a rustpreventive film of the second layer with a film thickness of 50 to 140 m is formed.