KNIVES is a system for providing balanced power supply and effective demand control. This system controls the power of electric devices over the Internet, due to reduce total power consumption. For the control, KNIVES also considers indoor environmental affinity by using environmental sensors. In this paper, CO2 sensor is implemented on KNIVES terminal and it enables to measure CO2 concentration for assessing indoor vitiation. This paper verifies the effectiveness of this sensor by estimating the number of people. Also, the number of people is one of the parameter for effective control. Existing estimating method for counting the number of people, however, is difficult to install in a existing building. This paper describes the new estimating method for the number of people by CO2 concentration. As a result, this method can accomplish accurate measurement, and it can be applied to various applications for a future demand control.