Structural body production method, structural body, and heat exchanger

The purpose of the present invention is to provide: a production method for a structural body, whereby the width of a welded section can be formed more narrowly during friction stir welding; a structural body; and a heat exchanger. In the production method for the structural body (1), an edge section (5) of the structural body (1) that is formed by superposing a plurality of members (2, 3) is welded by friction stir welding. This production method has: a welding step in which, a friction stir welding tool (7) is rotated and brought into contact with, from one surface side of a member (3) on one side, a superposed section of a workpiece (W) having the plurality of members (2, 3) superposed therein, and friction stir welded sections (6) are formed that are substantially the same as the shoulder diameter; and a cutting step in which the friction stir welded sections (6) are cut. Welded sections (61) of the edge section (5) in the structure are formed by the friction stir welded sections (6) that have been cut in each of at least two workpieces (W) obtained by cutting.