Abstract: A header tank for a heat exchanger includes tank forming sections that are joined into a tubular shape with a substantially square cross-section. The header tank has a refrigerant low path formed inside the tank forming sections include a tank upper section (31) in which tube insertion holes (31a) for connecting tubes of a heat exchanger core are formed. The tank lower section (33) having a substantially U-shaped refrigerant split-flow groove (33A) that has, in its longitudinal direction, communication holes (33a, 33b) for the refrigerant. The tank forming sections include a plate section (35) for closing an opening of the refrigerant split-flow groove (33A) to form a refrigerant split-flow path (37) inside the tank lower section.