A packing is held between a tank body and a header plate to secure sealing. Therewith, the width in a longer axis direction of an opening of a flat tube is wider and the width of the tank body is relatively narrow, resulting in a compact tank structure. A number of tooth parts protrude from the tank body at regular intervals, a tube end release part is formed between the tooth parts, and a side edge in a longitudinal axis direction of an opening of the flat tube is positioned therein. The tooth part, a flange part, a seal face of the header plate and a peripheral wall form a packing accommodation part, and a tip edge of the tooth part is seated on an edge of a convex part. The packing is arranged between the packing accommodation part and the seal face.