Time of Wetness CTOW), an index of moisture deterioration factors suffering building materials, was monitored by a dew detector: EKO MH-045 on south/30 deg tilted plane at outdoor exposure site in Tsukuba, Ibaraki, for a year. Ratio of Dewing is expressed as a multiTegression equation including relative humidity, ambient temperature, and wind speed/direction. Referring to the meteorological data observed around Japan for 1991-2000, yearly total of TOW was plotted on a map to be discussed on the local difference. Another estimated TOW based on previous ISO's criteria is less than those shown in this paper.