To increase a line speed, to improve productivity, to reduce costs and to efficiently form a uniform and excellent film.

The method is for forming a rustproof film on a PC steel stranded wire 1 in a series of lines. Heating is pre-heating before coating a synthetic resin powder coating material and post-heating after coating the synthetic resin powder coating material, and the temperature of the pre-heating is set 30-130°C higher than the temperature of the post-heating. In order to turn the resin film to a set film thickness, the synthetic resin powder coating material whose average grain size is 40-50 μm is used, and the speed of the line is turned to 5-10 m/min. Thus, not only the productivity is improved and the cost is reduced, but also the uniform and excellent film which does not damage flexibility and adhesion strength with concrete is efficiently formed.