A method of manufacturing a steel pipe, the method comprising: performing bending three or more times on a plate material along a width direction, the plate material being subjected to edge bending at both ends in the width direction, to form a preform having a U-shaped cross section; pressing the preform to form an open pipe, the open pipe being a tubular body having a seam gap portion in a longitudinal direction; and joining the seam gap portion to form a steel pipe, wherein when a width of the plate material before the edge bending is a plate width W, the preform has a lightly bent portion or an unbent portion of which center is positioned at a point away from a plate width end by W/4, the lightly bent portion having a small curvature compared with other regions, the unbent portion being not subjected to bending, and the pressing is performed to form the open pipe into a shape such that a range of 20 [%] or more of the plate width W of which center is positioned at a lowermost portion of the U-shaped cross section and a range of 10 [%] or more of the plate width W from the plate width end are inscribed in an arc with a diameter equal or substantially equal to an outer diameter of the steel pipe, the method being characterized in that the pressing is performed using a die having a radius of an arc portion within a range of ±3.5 [%] with respect to a radius corresponding to an outer radius of the steel pipe, and in that, before pressing the preform, in the U-shaped cross section of the preform, the angles θ 11 and θ 12 between a tangent TL 1 at a W/2 portion that is a plate width center portion and tangents TL 21 and TL 22 at the W/4 portions are 35 degrees or larger and smaller than 90 degrees, the angles θ 21 and θ 22 between tangents TL 31 and TL 32 at the plate width end portions and tangents TL 21 and TL 22 at the W/4 portions are 35 degrees or larger and smaller than 90 degrees, and the sum of the angles θ 11 and θ 12 between the tangent TL 1 and the tangents TL 21 and TL 22 is substantially equal to the sum of the angles θ 21 and θ 22 between the tangents TL 31 and TL 32 and the tangents TL 21 and TL 22. performing bending three or more times on a plate material along a width direction, the plate material being subjected to edge bending at both ends in the width direction, to form a preform having a U-shaped cross section; pressing the preform to form an open pipe, the open pipe being a tubular body having a seam gap portion in a longitudinal direction; and joining the seam gap portion to form a steel pipe, wherein when a width of the plate material before the edge bending is a plate width W, the preform has a lightly bent portion or an unbent portion of which center is positioned at a point away from a plate width end by W/4, the lightly bent portion having a small curvature compared with other regions, the unbent portion being not subjected to bending, and the pressing is performed to form the open pipe into a shape such that a range of 20 [%] or more of the plate width W of which center is positioned at a lowermost portion of the U-shaped cross section and a range of 10 [%] or more of the plate width W from the plate width end are inscribed in an arc with a diameter equal or substantially equal to an outer diameter of the steel pipe, the method being characterized in that the pressing is performed using a die having a radius of an arc portion within a range of ±3.5 [%] with respect to a radius corresponding to an outer radius of the steel pipe, and in that, before pressing the preform, in the U-shaped cross section of the preform, the preform has a lightly bent portion or an unbent portion of which center is positioned at a point away from a plate width end by W/4, the lightly bent portion having a small curvature compared with other regions, the unbent portion being not subjected to bending, and the pressing is performed to form the open pipe into a shape such that a range of 20 [%] or more of the plate width W of which center is positioned at a lowermost portion of the U-shaped cross section and a range of 10 [%] or more of the plate width W from the plate width end are inscribed in an arc with a diameter equal or substantially equal to an outer diameter of the steel pipe, the method being characterized in that the pressing is performed using a die having a radius of an arc portion within a range of ±3.5 [%] with respect to a radius corresponding to an outer radius of the steel pipe, and in that, before pressing the preform, in the U-shaped cross section of the preform, the angles θ 11 and θ 12 between a tangent TL 1 at a W/2 portion that is a plate width center portion and tangents TL 21 and TL 22 at the W/4 portions are 35 degrees or larger and smaller than 90 degrees, the angles θ 21 and θ 22 between tangents TL 31 and TL 32 at the plate width end portions and tangents TL 21 and TL 22 at the W/4 portions are 35 degrees or larger and smaller than 90 degrees, and the sum of the angles θ 11 and θ 12 between the tangent TL 1 and the tangents TL 21 and TL 22 is substantially equal to the sum of the angles θ 21 and θ 22 between the tangents TL 31 and TL 32 and the tangents TL 21 and TL 22.