A refrigerant circuit configured to perform a refrigerating cycle and comprising a heat exchanger, the heat exchanger comprising: a plurality of flat tubes; a first header-collecting pipe having inserted therein an end portion of each of the flat tubes; a second header-collecting pipe having inserted therein the other end portion of each of the flat tubes; and a plurality of fins joined to the flat tubes, wherein the second header-collecting pipe forms flow spaces (71a-71c) which communicate with the plurality of flat tubes and in which a refrigerant being in a gas-liquid two-phase state flows upwardly when the heat exchanger functions as an evaporator, an effective cross-sectional area of the flow spaces (71a-71c) is an area obtained by subtracting a projected area which corresponds to a portion of each flat tube located in a corresponding one of the flow spaces (71a-71c) and which is projected onto a plane perpendicular to an axial direction of the second header-collecting pipe, from an area of a cross section of the corresponding one of the flow spaces (71a-71c) which is perpendicular to the axial direction of the second header-collecting pipe, characterized in that the effective cross-sectional area of the flow spaces (71a-71c) is set based on a mass flow rate at which the refrigerant flows into the flow spaces (71a-71c) when the heat exchanger functions as the evaporator; and a value included in a variation range of the mass flow rate at which the refrigerant flows into the flow spaces (71a-71c) when the heat exchanger functions as the evaporator is determined as a reference mass flow rate M R [kg/h], and the effective cross-sectional area A [mm 2] of the flow spaces (71a-71c) is equal to or greater than (1.91M R - 22.7) and equal to or smaller than (1.96M R + 30.8), or is equal to or greater than (1.96M R - 25.0) and equal to or smaller than (1.96M R + 30.0). a plurality of flat tubes; a first header-collecting pipe having inserted therein an end portion of each of the flat tubes; a second header-collecting pipe having inserted therein the other end portion of each of the flat tubes; and a plurality of fins joined to the flat tubes, wherein the second header-collecting pipe forms flow spaces (71a-71c) which communicate with the plurality of flat tubes and in which a refrigerant being in a gas-liquid two-phase state flows upwardly when the heat exchanger functions as an evaporator, an effective cross-sectional area of the flow spaces (71a-71c) is an area obtained by subtracting a projected area which corresponds to a portion of each flat tube located in a corresponding one of the flow spaces (71a-71c) and which is projected onto a plane perpendicular to an axial direction of the second header-collecting pipe, from an area of a cross section of the corresponding one of the flow spaces (71a-71c) which is perpendicular to the axial direction of the second header-collecting pipe, characterized in that the effective cross-sectional area of the flow spaces (71a-71c) is set based on a mass flow rate at which the refrigerant flows into the flow spaces (71a-71c) when the heat exchanger functions as the evaporator; and a value included in a variation range of the mass flow rate at which the refrigerant flows into the flow spaces (71a-71c) when the heat exchanger functions as the evaporator is determined as a reference mass flow rate M R [kg/h], and