An air-conditioning device for a vehicle having a vehicle cabin, comprising: a duct configured to feed air to the vehicle cabin; a blower, which is arranged in the duct, and is configured to generate airflow toward the vehicle cabin; a cooling device configured to cool air passing through the duct; a coolant circulation system supplying an engine coolant; a heater core, which is arranged in the duct on a downstream side of the airflow with respect to the cooling device, and is configured to use the engine coolant supplied by the coolant circulation system as a heat source to heat the passing air; an air mix damper configured to change a ratio between an air amount passing through the heater core and an air amount bypassing the heater core; a water valve, which is provided in the coolant circulation system on an upstream side of the heater core, and is capable of changing an opening amount; and a control device configured to control the blower, the cooling device, the air mix damper, and the water valve, wherein the control device is configured to decrease the opening amount of the water valve in a predetermined cooling mode; and the control device is configured to decrease cooling performance of the cooling device in a case where the opening amount of the water valve is decreased compared with a case where the opening amount of the water valve is not decreased and wherein the cooling device comprises: an electric compressor configured to compress a refrigerant to discharge the compressed refrigerant; an evaporator, which is arranged in the duct on a downstream side of the airflow with respect to the blower, and is configured to evaporate the refrigerant sucked into the electric compressor; and a condenser configured to condense the refrigerant evaporated by the evaporator; the control device is configured to control the electric compressor and the condenser so that a temperature of the evaporator reaches a target temperature; and the control device is configured to decrease the cooling performance of the cooling device through at least one of an increase in the target temperature, or a decrease in an upper limit rotational speed of the electric compressor, when the opening amount of the water valve is decreased. a duct configured to feed air to the vehicle cabin; a blower, which is arranged in the duct, and is configured to generate airflow toward the vehicle cabin; a cooling device configured to cool air passing through the duct; a coolant circulation system supplying an engine coolant; a heater core, which is arranged in the duct on a downstream side of the airflow with respect to the cooling device, and is configured to use the engine coolant supplied by the coolant circulation system as a heat source to heat the passing air; an air mix damper configured to change a ratio between an air amount passing through the heater core and an air amount bypassing the heater core; a water valve, which is provided in the coolant circulation system on an upstream side of the heater core, and is capable of changing an opening amount; and a control device configured to control the blower, the cooling device, the air mix damper, and the water valve, wherein the control device is configured to decrease the opening amount of the water valve in a predetermined cooling mode; and the control device is configured to decrease cooling performance of the cooling device in a case where the opening amount of the water valve is decreased compared with a case where the opening amount of the water valve is not decreased and wherein the cooling device comprises: an electric compressor configured to compress a refrigerant to discharge the compressed refrigerant; an evaporator, which is arranged in the duct on a downstream side of the airflow with respect to the blower, and is configured to evaporate the refrigerant sucked into the electric compressor; and a condenser configured to condense the refrigerant evaporated by the evaporator; an electric compressor configured to compress a refrigerant to discharge the compressed refrigerant; an evaporator, which is arranged in the duct on a downstream side of the airflow with respect to the blower, and is configured to evaporate the refrigerant sucked into the electric compressor; and a condenser configured to condense the refrigerant evaporated by the evaporator; the control device is configured to control the electric compressor and the condenser so that a temperature of the evaporator reaches a target temperature; and the control device is configured to decrease the cooling performance of the cooling device through at least one of an increase in the target temperature, or a decrease in an upper limit rotational speed of the electric compressor, when the opening amount of the water valve is decreased.