A method for installing a refrigerant radiator in a vehicle, the refrigerant radiator being part of a vapor-compression refrigerant cycle and being adapted to exchange heat between a high-temperature and high-pressure refrigerant compressed by a compressor of the vapor-compression refrigerant cycle and air blown into a space for air conditioning to thereby radiate heat from a gas-phase refrigerant having a degree of superheat so as to transfer the gas-phase refrigerant to a liquid-phase refrigerant having a degree of supercooling, the refrigerant radiator comprising: a refrigerant, a plurality of tubes allowing the refrigerant to flow therethrough, wherein the tubes extend in a direction perpendicular to a horizontal direction, or at an angle with respect to the horizontal direction for allowing the refrigerant to flow therethrough from a lower side to an upper side, wherein the refrigerant radiator is installed so that in use the tubes satisfy the following relationship: Re ≥ A × X 6 + B × X 5 + C × X 4 + D × X 3 + E × X 2 + F × X + G A = − 0.0537 × θ 2 + 9.7222 × θ + 407.19 B = − − 0.2093 × θ 2 + 37.88 × θ + 1586.3 C = − 0.3348 × θ 2 + 60.592 × θ + 2538.1 D = − − 0.2848 × θ 2 + 51.53 × θ + 2158.2 E = − 0.1402 × θ 2 + 25.365 × θ + 1062.8 F = − − 0.0418 × θ 2 + 7.5557 × θ + 316.46 G = − 0.0132 × θ 2 + 2.3807 × θ + 99.73 wherein θ (°) is an inclination angle formed by a flow direction of the refrigerant flowing through the tube and the horizontal direction; X is a dryness of the refrigerant in a predetermined position where the refrigerant flowing through the tube is a gas-liquid two-phase refrigerant; and Re is a Reynolds number of the refrigerant in the predetermined position that is determined from an average flow velocity (m/S) of the refrigerant flowing through the tube, and wherein the inclination angle (θ) is in a range of more than 0 ° and not more than 90 ° (0 < θ ≤ 90 °). a refrigerant, a plurality of tubes allowing the refrigerant to flow therethrough, wherein the tubes extend in a direction perpendicular to a horizontal direction, or at an angle with respect to the horizontal direction for allowing the refrigerant to flow therethrough from a lower side to an upper side, wherein the refrigerant radiator is installed so that in use the tubes satisfy the following relationship: Re ≥ A × X 6 + B × X 5 + C × X 4 + D × X 3 + E × X 2 + F × X + G A = − 0.0537 × θ 2 + 9.7222 × θ + 407.19 B = − − 0.2093 × θ 2 + 37.88 × θ + 1586.3 C = − 0.3348 × θ 2 + 60.592 × θ + 2538.1 D = − − 0.2848 × θ 2 + 51.53 × θ + 2158.2 E = − 0.1402 × θ 2 + 25.365 × θ + 1062.8 F = − − 0.0418 × θ 2 + 7.5557 × θ + 316.46 G = − 0.0132 × θ 2 + 2.3807 × θ + 99.73 wherein θ (°) is an inclination angle formed by a flow direction of the refrigerant flowing through the tube and the horizontal direction; X is a dryness of the refrigerant in a predetermined position where the refrigerant flowing through the tube is a gas-liquid two-phase refrigerant; and Re is a Reynolds number of the refrigerant in the predetermined position that is determined from an average flow velocity (m/S) of the refrigerant flowing through the tube, and wherein the inclination angle (θ) is in a range of more than 0 ° and not more than 90 ° (0 < θ ≤ 90 °).