Estimating the number of visitors in museums is an important issue. This paper proposes to solve this problem by measuring CO_2 concentration in a museum. The CO_2 concentration is mainly affected by the number of visitors as well as the size of space, the amount of ventilation and the time to stay. A simplified model for estimating the number of visitors from the CO_2 measurement was investigated and applied at MUSEUM OF CONTEMPORARY ART TOKYO. Experimental results show that our proposed method can estimate the number of visitors in a day within 6.7% of errors.