An air-conditioning system comprising: an opening/closing device configured to cause an openable portion provided in an opening of a building to switch between an open state of allowing movement of air between an inside and an outside of the building, and a closed state; an air-conditioning device configured to perform air-conditioning inside the building by consuming energy for operation; a monitoring device configured to monitor environmental information regarding the inside and the outside of the building; and a controller configured to control the opening/closing device and the air-conditioning device based on the environmental information obtained from the monitoring device, and draft information determined by parameters including an opening area and an opening shape in the openable portion, characterized in that the controller is configured to obtain a Predicted Mean Vote, PMV, index outside the building and a PMV index inside the building based on the environmental information, wherein the controller is configured to select an energy saving mode of setting the openable portion to the open state and stopping the air-conditioning device, when the PMV index outside the building is smaller than an the PMV index inside the building, and wherein the controller is configured to select an air-conditioning mode of setting the openable portion to the closed state and starting the air-conditioning device, when the PMV index inside the building is smaller than the PMV index outside the building; wherein the controller is configured to control the opening/closing device and the air-conditioning device further based on date and time information representing a current date and time, and building information representing a location condition of the building; wherein the controller further has a cooperation mode of setting the openable portion to the open state and starting the air-conditioning device, wherein the controller is configured to determine, in said cooperation mode, an opening degree of the openable portion and the desired temperature of the air-conditioning device based on the environmental information. an opening/closing device configured to cause an openable portion provided in an opening of a building to switch between an open state of allowing movement of air between an inside and an outside of the building, and a closed state; an air-conditioning device configured to perform air-conditioning inside the building by consuming energy for operation; a monitoring device configured to monitor environmental information regarding the inside and the outside of the building; and a controller configured to control the opening/closing device and the air-conditioning device based on the environmental information obtained from the monitoring device, and draft information determined by parameters including an opening area and an opening shape in the openable portion, characterized in that the controller is configured to obtain a Predicted Mean Vote, PMV, index outside the building and a PMV index inside the building based on the environmental information, wherein the controller is configured to select an energy saving mode of setting the openable portion to the open state and stopping the air-conditioning device, when the PMV index outside the building is smaller than an the PMV index inside the building, and wherein the controller is configured to select an air-conditioning mode of setting the openable portion to the closed state and starting the air-conditioning device, when the PMV index inside the building is smaller than the PMV index outside the building; wherein the controller is configured to control the opening/closing device and the air-conditioning device further based on date and time information representing a current date and time, and building information representing a location condition of the building; wherein the controller further has a cooperation mode of setting the openable portion to the open state and starting the air-conditioning device, wherein the controller is configured to determine, in said cooperation mode, an opening degree of the openable portion and the desired temperature of the air-conditioning device based on the environmental information.