One of the external factors that affect human intellectual activity is the concentration of carbon dioxide in the environment. Previous studies have measured individual carbon dioxide concentrations in a room, but it is not clear how carbon dioxide concentrations change in the case of multiple individuals. In this study, we used a small sensor device to simultaneously measure the carbon dioxide concentration in a room with multiple people. Furthermore, by comparing the predicted values using existing prediction models with the measured values, we will verify whether the prediction models can be used effectively in this measurement method. The experimental results show that the carbon dioxide generated by human exhalation diffuses regardless of the distance from the devices, the height difference, or the placement of the person. It was shown that the existing model for predicting carbon dioxide concentration was sufficiently useful for rooms with more than a certain number of ventilations, but in rooms with fewer ventilation, there was an error between the measured value and the actual value.