A square pipe manufacturing method includes: a first processing step of forming a first intermediate formed product 13 by bending both widthwise end portions of a rectangular metal plate 1; a second processing step of forming a second intermediate formed product 21 by bending both widthwise sides of a bottom surface 11 of the first intermediate formed product 13; a third processing step of obtaining a third intermediate formed product 23 by curving the side wall surfaces 17 to be convex outward in the width direction by pressing the flange parts 7, 7 toward the bottom wall surface 19 and then pressing the curved side wall surfaces 17 inward in the width direction; and a fourth processing step of crush-deforming corners 31, 31, 33, 33 of the third intermediate formed product 23.