An air conditioning system includes: an air conditioner to regulate a state of air in a room; a sensor device including at least one detector to detect an amount of an odor-causing substance included in the air in the room; and a control device to control operation of the air conditioner. The control device includes: a first acquisition unit to acquire a detected value of each of the at least one detector at a first timing when a person is present in the room; a second acquisition unit to acquire a detected value of each of the at least one detector at a second timing later than the first timing; a difference calculator to calculate a difference between the detected value at the first timing and the detected value at the second timing; and an instruction unit to control operation of the air conditioner based on the difference.