For the people who live in cold region, residence quality is especially important to their health, not only from economical aspects such as heating costs in the winter, but also thermal sanitation. Thatched houses are not rare in the district areas. From the climatic or historical implication, old thatched houses are recently improved or rebuilt and people intend to use them. That is, improvements are carried out on such old houses, and the good points are utilized, and reconstruction undertaken, and the example of presenting the house adaptation is also mainly observed. In this study, the thermal sanitation of the indoor environment in the winter was mainly examined on the cases of thatched houses near Fukushima City. The thatched houses used as research objects, are over 100 years after construction and they were previously used as sericulture farmhouses. A temperature and humidity collector and an automatic recording thermo-hygrometer were used in the measurement of the air temperature and humidity. This study was carried out in winter, February and March. The temperature was considerably different according to various parts of the rooms and time, and the air temperature near the ceiling was higher generally than the lower parts of room, near the floor. In the research of the housing where large-scale reconstruction was made, the room temperatures of the living room and kitchen were comparatively high at the time when they were used. In the meantime, room temperatures in the washroom, bedroom, etc. were generally low, showing a changing pattern, which was almost parallel to the change of the outside ambient temperature. In the research of the housing where ceilings had been added and the hearth, etc. had been improved; the room temperatures of most rooms were the lowest in early the morning. The room temperature of the living room rose in the morning and evening where the family gathered, and it was especially higher in the evening, showing air temperature from 12 degrees C to 18 degrees C. In the bedrooms, which did not use heating appliances such as oil heaters etc., the room temperature was generally low. In the present study, the room temperature of the living room was comparatively high and that of the bedrooms and lavatories in houses with inadequate heating system was low. Even if reconstruction or improvement of old houses is made, the thermal indoor condition is not sufficient. Consideration of thermal facilities of floor heating, etc. is necessary with the air-tightness of houses following reconstruction.