A heat exchanger has a flat tube (10) having a curved portion (13). A part of the curved portion (13) is formed by overlapping an outer rim (22) on an inner rim (21). The inner rim (21) has a small curvature region (102). The small curvature region (102) is inclined to a flat plate portion (11), and is defined by a radius larger than a difference between a half of a thickness of the flat tube (10) and a thickness of the outer rim (22). The small curvature region (102) is not beyond a center line (C1) in a thickness of the flat tube (10). The outer rim (22) extends beyond the center line (C1). The outer rim (22) has an end face (22a) placed on the small curvature region (102). The flat tube (10) has flared portions (15, 16) expanded at insertion holes (54).