A steel strip is formed into a cylinder so that opposite ends of the steel strip face each other, while continuously conveying the steel strip. The opposite ends are melted across the entire thickness by irradiating the opposite ends with a high-energy-density beam while applying a pressure with squeeze rolls. A seam is formed by joining the opposite ends, and excess weld metal of the seam on the outer and inner sides of the obtained welded steel pipe is removed by cutting. A repaired seam is then formed by remelting and solidifying a region on the inner side of the welded steel pipe, the region having a depth of 0.5 mm or more from the surface and a width of double or more the width of the seam, and in addition, the center line of the seam is made to coincide with the center line of the repaired seam.