<p> In welfare facilities for the elderly, from the viewpoint of hygiene, a lot of attention is paid to ventilation and humidification. However, excessive ventilation in winter promotes the inflow of outdoor air with low temperature and humidity, and brings low humidity, cold draft, and increase in heating load. Furthermore, the airflow route in a facility should be designed to minimize infection or odor diffusion. It is important to grasp the airflow distribution of whole the building quantitatively. However, it is not easy to clarify it by measurements, especially for building with a large scale. In this study, measurements of indoor environment on site along with the multiple interzonal airflow analysis were conducted for a welfare facility for the elderly in Hokkaido. The distributions of air temperature, humidity and CO₂ concentration in the building were measured through a winter. As the results, it was shown that the indoor absolute humidity and CO₂ concentration became decreased during the operation of an exhaust fan in the kitchen with a large capacity. The decrease occurred even in a room far from the kitchen, and thus it was suggested that the exhausting air from the kitchen predominated the indoor environment of the whole building, and that the air from various spaces in the facility was gathered to the dayroom located next to the kitchen. In addition, an interzonal airflow analysis was performed for this whole building to calculate airflow distribution for a representative day in winter, considering the estimated opening areas, the operating time and airflow rate of all the exhaust fans including kitchen, toilet, bathroom, and the other rooms in the facility, with the air temperature distribution determined based on measured values. The CO₂ concentration and absolute humidity balance equations were solved simultaneously, by determining generation rate from persons (CO₂ and moisture) and humidifier and bathroom (moisture). The calculated results of distributions of CO₂ concentration and absolute humidity were shown to agree well with the measure values. Based on the calculated results of this validated model, it was shown that the air quality of each bedroom satisfied a proper level. The air in the day service center significantly flow into the dayroom of the inhabitants of the facility, which was not proper from the viewpoint of infection control. On the other hand, during nighttime when the exhaust fan in the kitchen was stopped, the airflow was balanced within several blocks dominated by exhaust fans of toilet operated continuously. At last, analysis for two improved cases in ventilation operation were performed; a case of operating the kitchen fan intermittently and a case introducing direct air supply into kitchen equivalent with the exhaust airflow rate of the kitchen fan. As the results, it was shown that the increase in humidity, and saving in heat load are possible within an acceptable levels of increase in CO₂ concentration by intermittent operation in the kitchen and introducing direct air supply to the kitchen. Furthermore, in the latter case, although decrease in heat loss by air infiltration into building will be cancelled by heating the air supplied to kitchen, the airflow was balanced within several blocks in the building, which is a merit from the viewpoint of controls of infection or odor diffusion in the facility.</p>