It is important that the countermeasures to reduce CO2 emission during the housing usage phase be examined. In this study, the factors under various daily activities concerning the CO2 emission were simulated, in order to examine a more effective countermeasure. Then, it was confirmed that CO2 emission, that can be controlled by residents, occupies a large portion. For this reason, it was also found that the life style influenced the amount of CO2 emission considerably. These results suggest that the improvement of the residential life style is important for the general reduction of the CO2 emission.