An air conditioning system configured to perform cooling operation in an air conditioning target space, the air conditioning system comprising: an air conditioner including a heat source unit and a plurality of utilization units constituting a single refrigerant circuit system; a ventilator; and a controller (29, 36) configured to control the air conditioner and the ventilator, wherein the plurality of utilization units is divided at least into a first utilization unit that is installed in a first air conditioning target space and belongs to a first group, and a second utilization unit that is installed in a second air conditioning target space and belongs to a second group, the first air conditioning target space and the second air conditioning target space each serving as the air conditioning target space, the ventilator is included in the first group and is configured to ventilate air for the first air conditioning target space, the first utilization unit includes a heat exchanger, the second utilization unit includes a heat exchanger, the first group includes a valve configured to adjust a flow rate of a refrigerant flowing to at least one heat exchanger of the first utilization unit, and the second group includes a valve configured to adjust a flow rate of a refrigerant flowing to at least one heat exchanger of the second utilization unit, characterized in that, when, as to the first group, an outdoor air condition is satisfied for outdoor air, which is air outside the air conditioning target space, the controller (29, 36) is configured to close the valve of the first group and cause the ventilator of the first group to perform outdoor air cooling operation to introduce the outdoor air into the first air conditioning target space, and configured to keep controlling an opening degree of the valve of the second group while operating the heat source unit so that the cooling operation by the air conditioner continues in the second air conditioning target space. an air conditioner including a heat source unit and a plurality of utilization units constituting a single refrigerant circuit system; a ventilator; and a controller (29, 36) configured to control the air conditioner and the ventilator, wherein the plurality of utilization units is divided at least into a first utilization unit that is installed in a first air conditioning target space and belongs to a first group, and a second utilization unit that is installed in a second air conditioning target space and belongs to a second group, the first air conditioning target space and the second air conditioning target space each serving as the air conditioning target space, the ventilator is included in the first group and is configured to ventilate air for the first air conditioning target space, the first utilization unit includes a heat exchanger, the second utilization unit includes a heat exchanger, the first group includes a valve configured to adjust a flow rate of a refrigerant flowing to at least one heat exchanger of the first utilization unit, and the second group includes a valve configured to adjust a flow rate of a refrigerant flowing to at least one heat exchanger of the second utilization unit, characterized in that, when, as to the first group, an outdoor air condition is satisfied for outdoor air, which is air outside the air conditioning target space, the controller (29, 36) is configured to close the valve of the first group and cause the ventilator of the first group to perform outdoor air cooling operation to introduce the outdoor air into the first air conditioning target space, and configured to keep controlling an opening degree of the valve of the second group while operating the heat source unit so that the cooling operation by the air conditioner continues in the second air conditioning target space.