To solve problems in conventional technology for spraying Zn in a state of directing a Zn thermal spraying gun to both flat faces of a tube, that the amount of Zn attached to a tip part in the width direction of the tube is small, which may cause the corrosion of the tip part when it is used for a long time, and the equipment cost is increased when the Zn thermal spraying gun is installed more.

In a Zn thermal spraying process, Zn is sprayed in a state that the Zn thermal spraying gun 10 is directed to both flat faces 2a of the tube 2, and Zn of 10 -18 g/m2 is thermally sprayed to the flat faces 2a, to attach Zn to the flat faces 2a with an attachment area ratio of 70 % or more, and to the tip part 2b with an attachment area ratio of 60 % or more. By increasing the Zn thermal spraying amount to the flat faces 2a, the corrosion of the tip part 2b can be also prevented. Further as the corrosion of the tip part 2b can be prevented by increasing the Zn thermal spraying amount without adding the Zn thermal spraying gun 10, the equipment cost is not increased.