The effect of improving indoor environment by the cross ventilation, the rate to discharge thermal energy by the ventilation is larger than the reduction effects of SET* by indoor air flow. In this study, the relationship between the difference of indoor temperature and outdoor temperature by the effect of ventilation will be less than 1°C and the opening area of windows ratio are calculated for a simple and the standerd AIJ detached house model. The relationship between the air change rate, opening area of windows conditions and cross ventilation performance of the house is clarified.The results are as follows; (1) When air change rate is 20 times/h, the inside air temperature becomes nearly equal to the outside air temperature due to the discharge thermal energy by the ventilation. (2) The opening area of windows ratio tends to be high in the inland, and it tends to be low in the city along the sea in Japan. (3) Results of standard AIJ detached house model, the opening area of windows ratio with which ventilation rate is 20 times/h becomes a low value of feasibility. The opening area of windows ratio with which ventilation rate is 10 times/h becomes a values that may be realized.