An air conditioning system includes: a sensor; an obtainer that obtains information indicating air quality in an indoor space, which is output by the sensor; and a controller that calculates an air quality index based on the information obtained, and controls a ventilator, based on the air quality index calculated. The air quality index is expressed by f(Xn)×g(T, H), where f(Xn) denotes a function with respect to Xn that indicates at least one of a CO2 concentration, a concentration of total volatile organic compounds (TVOC), a concentration of particulate matter (PM), an NOX concentration, an SOX concentration, an O3 concentration, a mold count, or a dust count, and g(T, H) denotes a function with respect to T denoting temperature and H denoting humidity in the indoor space.