Recently, many studies dealing with energy saving have been conducted from the viewpoint of mitigating global warming. However, most of them only consider energy saving and ignore the total environmental impact. It is necessary to provide a comfortable work space using efficient and optimized energy control, because excessive demand control can have a negative impact on the environment. It can also have a negative impact on working or learning efficiency. To provide a comfortable indoor environment, it is important to measure environmental indexes, including not only temperature, humidity, illuminance, and wind speed, but also CO2 concentration. Moreover, to forecast air conditioner demand, it is important to know the number of people in the controlled space. In this paper, we propose a new method to estimate the number of people in a ventilation-controlled environment by using a CO2 sensor. The proposed method will provide a demand control system that achieves more intelligent and accurate control of ventilation fans.