Traditional housing units like the ones in Japan, China, and Cambodia are naturally ventilated. Although the windows of contemporary houses are getting smaller to ensure maximum insulation and airtightness, natural ventilation ought to be utilized more for healthy buildings. This is true not only for office buildings but also for housing units. The purpose of this research was to test the operating method and efficiency of a newly developed natural ventilation window for houses. To do this, a field survey was conducted in a detached house with natural ventilation windows. The factors investigated were the thermal environment of the house, the airflow volume through the natural ventilation windows, and window utilization. The following results were obtained: (1) The windows were well opened in the temperature range of 19-25°C. (2) The evaluation of the thermal comfort during naturally ventilation by using an adaptive model showed that the mean indoor operative temperature was coincident with the neutral temperature of the adaptive model.