In a heat exchanger, each of flat tubes (31) has an end portion inserted in a header-collecting pipe (70). When the heat exchanger functions as an evaporator, a refrigerant in a gas-liquid two-phase state upwardly flows in a subspace (71 a) located in the header-collecting pipe (70). An effective cross-sectional area A of the subspace (71 a) in the header-collecting pipe (70) is set based on a mass flow rate of the refrigerant flowing into the subspace (71a) in the header-collecting pipe (70). The effective cross-sectional area A is obtained by subtracting a projected area A 1 which corresponds to a portion of each flat tube (31) located in the subspace (71a) and is projected onto a plane perpendicular to an axial direction of the header-collecting pipe (70) from an area A 0 of a cross section of the subspace (71 a) which is perpendicular to the axial direction of the header-collecting pipe (70).
