A control method for a grille device for a vehicle arranged at a head end part of the vehicle and having a grille opening an opening degree of which is capable of being adjusted between the minimum opening degree and the maximum opening degree, in which the vehicle includes an air conditioner for a vehicle, the grille device for the vehicle, the grille controller controlling the grille device for the vehicle and a radiator cooled by outside air introduced from the grille opening, the air conditioner for the vehicle includes a refrigeration cycle having a refrigerant circuit connecting a compressor, a condenser, an expansion device and an evaporator by pipes (61, 62, 63, 64, 65, 66) to circulate a refrigerant, and an internal heat exchanger performing heat exchange of the refrigerant between a first heat exchange part in which the refrigerant introduced from the condenser to the expansion device flows and a second heat exchange part in which the refrigerant introduced from the evaporator to a suction side of the compressor flows, in which the condenser is cooling the refrigerant in the high-temperature and high-pressure vaporized state discharged from the compressor 2 using outside air in front of the vehicle introduced from the grille opening 21 by any one of or both of the running of the vehicle or the operation of a cooling fan 7 so that the refrigerant becomes in a high-temperature and high-pressure liquid state the air conditioner for the vehicle includes a first temperature sensor detecting a temperature of the evaporator or a temperature of air passing through the evaporator, or a second temperature sensor having a measurement point between the evaporator and the internal heat exchanger and an output part outputting a detected value detected by the first temperature sensor or the second temperature sensor to a grille controller, the control method comprising: Step S11 of determining the opening degree at the time of determination by the grille controller; Step S12 of comparing the detected value with a prescribed threshold value by the grille controller; and Step S13 of controlling the grille opening by the grille controller, characterized in that the Step S13 includes Step S131 of calculating an ideal opening degree necessary for cooling the condenser, Step S132 of calculating a necessary opening degree necessary for cooling the radiator and Step S133 of comparing the ideal opening degree with the necessary opening degree, when the ideal opening degree exceeds the necessary opening degree, control of setting the grille opening to the ideal opening degree is performed, and when the ideal opening degree is equal to or lower than the necessary opening degree, control of setting the grille opening to the necessary opening degree is performed, and the ideal opening degree satisfies the following condition 1, Condition 1: When the opening degree at the time of determination is the maximum opening degree and the detected value is lower than a first threshold value, the ideal opening degree is smaller than the maximum opening degree, and when the opening degree at the time of determination is the minimum opening degree and the detected value is equal to or higher than the first threshold value or exceeds a second threshold value different from the first threshold value, the ideal opening degree is larger than the minimum opening degree. Step S11 of determining the opening degree at the time of determination by the grille controller; Step S12 of comparing the detected value with a prescribed threshold value by the grille controller; and Step S13 of controlling the grille opening by the grille controller, characterized in that the Step S13 includes Step S131 of calculating an ideal opening degree necessary for cooling the condenser, Step S132 of calculating a necessary opening degree necessary for cooling the radiator and Step S133 of comparing the ideal opening degree with the necessary opening degree, when the ideal opening degree exceeds the necessary opening degree, control of setting the grille opening to the ideal opening degree is performed, and when the ideal opening degree is equal to or lower than the necessary opening degree, control of setting the grille opening to the necessary opening degree is performed, and the ideal opening degree satisfies the following condition 1, Condition 1: When the opening degree at the time of determination is the maximum opening degree and the detected value is lower than a first threshold value, the ideal opening degree is smaller than the maximum opening degree, and when the opening degree at the time of determination is the minimum opening degree and the detected value is equal to or higher than the first threshold value or exceeds a second threshold value different from the first threshold value, the ideal opening degree is larger than the minimum opening degree.