The air-conditioning system includes: an opening/closing device opening and closing an openable portion in an opening of a building; an air-conditioning device performing air-conditioning inside the building by consuming energy for operation; a monitoring device monitoring environmental information regarding the inside and outside of the building; and a controller controlling the opening/closing device and the air-conditioning device based on the environmental information from the monitoring device, date and time information representing a current date and time, building information representing a location condition of the building, and draft information determined by parameters including an opening area and an opening shape in the openable portion. The controller opens the openable portion and stops the air-conditioning device when the outside is more favorable in air environment than the inside, and closes the openable portion and starts the air-conditioning device when the inside is more favorable in air environment than the outside.