A method of manufacturing a welded steel pipe, the method comprising: performing three-point bending press forming on a raw material steel plate by a pair of dies disposed in a steel plate feeding direction at a predetermined gap, and a punch configured to press the steel plate between the pair of dies to form an open pipe; and welding the open pipe, wherein, when the open pipe is formed such that after performing first half press forming from one width end portion of the steel plate toward a width center, provided leaving the width center unpressed, second half press forming is performed from a width end portion of an opposite side toward the width center, provided leaving the width center unpressed, and the width center is finally subjected to final press forming, the steel pipe, supported on the dies of the steel plate width center side, is assumed to be a non-formed portion in a final pass of the second half press forming, characterized in that the final pass of the second half press forming satisfies the following Formula: β b ⋅ W < α b ⋅ L b + L n where, L b represents forming range (mm) in the final pass of the second half press forming, L n represents forming range (mm) in the final press forming, W represents die gap (mm), α b represents shift ratio (-) of steel plate position in the final pass of the second half press forming, and β b represents shift ratio (-) of die position in the final pass of the second half press forming. performing three-point bending press forming on a raw material steel plate by a pair of dies disposed in a steel plate feeding direction at a predetermined gap, and a punch configured to press the steel plate between the pair of dies to form an open pipe; and welding the open pipe, wherein, when the open pipe is formed such that after performing first half press forming from one width end portion of the steel plate toward a width center, provided leaving the width center unpressed, second half press forming is performed from a width end portion of an opposite side toward the width center, provided leaving the width center unpressed, and the width center is finally subjected to final press forming, the steel pipe, supported on the dies of the steel plate width center side, is assumed to be a non-formed portion in a final pass of the second half press forming, characterized in that the final pass of the second half press forming satisfies the following Formula: β b ⋅ W < α b ⋅ L b + L n where, L b represents forming range (mm) in the final pass of the second half press forming, L n represents forming range (mm) in the final press forming, W represents die gap (mm), α b represents shift ratio (-) of steel plate position in the final pass of the second half press forming, and β b represents shift ratio (-) of die position in the final pass of the second half press forming. where, L b represents forming range (mm) in the final pass of the second half press forming, L n represents forming range (mm) in the final press forming, W represents die gap (mm), α b represents shift ratio (-) of steel plate position in the final pass of the second half press forming, and β b represents shift ratio (-) of die position in the final pass of the second half press forming.