To improve corrosion resistance and weatherability of a PC-steel stranded wire to solve the problem that a rustproof film formed on a conventional PC-steel stranded wire is inferior in a weatherability environment.

A method of forming the rustproof film of the PC-steel stranded wire 1 includes: unstranding the PC-steel stranded wire 1 to loosen a side wire from a core wire; applying a synthetic resin powder coating to the respective outer peripheral surfaces of the core wire and the side wire in a loosened state, heating to stick evenly, and then cooling to form the rustproof film; then stranding the side wire around the core wire to the original state. In the method, after the primary coating film and secondary coating film of different properties are melted, bonded, integrated, and hardened, the side wire is stranded around the core wire to the original state, and PC-steel with improved corrosion resistance and weatherability is obtained.