The <i>Santono-dai</i> site is located at Yokohama city, near Tokyo. More than 250 dwelling places in the Jomon, Yayoi, and Kofun periods were excavated at the site in 1961. The site was designated as an important national site. Most of the dwelling places were buried again after examination but some remained open in a shelter for public display. Because of high humidity, about 80% relative humidity even in winter, dwelling places in the shelter are now covered by molds, mosses, and lichens. The measurement of temperature and humidity by a data logger revealed that the humidity rose mainly at a sunny place in the shelter. It was also proved by a moisture flux meter that evaporation from the ground increased rapidly when sun shone through a window. The authors recommend the use of an artificial cool light for illumination.