In Japan, natural ventilation through large openings has been used for cooling in summer and medium season. However, air conditioner has become popular recently, because it is hot and humid in summer in most parts of Japan. It causes cooling energy consumption to rise continuously. Nowadays, as global warming has become a serious problem, natural ventilation has been considered as a key method for cooling energy conservation. However, there is insufficient knowledge of natural ventilation effectiveness on cooling energy conservation quantitatively. Therefore, in order to verify the effectiveness of natural ventilation through large openings, we have been conducting experiments by automatically simulating various occupants’ behaviors, namely cooking, lighting, usage of electric appliances and domestic hot water, operation of air conditioners, windows and curtain opening/closing. In this paper, we have performed experiments with additional several patterns, which have different thermal conditions for on operation of air conditioners, to our previous paper. And based on the experimental results, we analyzed the heat discharge by natural ventilation and the cooling capacity of air conditioner. The result suggests that though natural ventilation sometimes brings the gain of sensible and latent heat, it is not so much as to increase the daily cooling capacity of air conditioner.