This paper described the field measurement results of the local mean age of air which was defined the time of air reaching to the measuring points, and the local air change effectiveness in the wooden detached house. The evaluation of infiltration is indispensable because there are many variation of the infiltration (natural ventilation) in comparison with the office building. However, it is difficult to evaluate the amount of infiltration quantitatively as for the infiltration because it changes by the difference in temperature outside the inside and the wind speed and direction. When ventilation is evaluated by the measurement of local mean age of air, they must evaluate local mean age of air of the amount and change of infiltration. The air change effectiveness changed in the amount of infiltration and age of air of the infiltration were put in the consideration was examined which was by this paper. As a result, how to calculate the amount of infiltration of every measurement point by using response factor method with the outside wind speed and a difference in indoor and outside temperature was tried, and have a good result which get age of infiltration air.
