Behavior of precipitable water vapor (PWV) routinely retrieved from the nationwide array of the Global Positioning System (GPS) established by Geographical Survey Institute (GSI) of Japan are compared with the Japan area objective analysis data for numerical weather prediction (NWP) of the Japan Meteorological Agency (JMA). The array used here has a spatial resolution of about 50 km for monitoring crustal deformation. The 3-hourly zenith tropospheric delay (ZTD) data obtained in GSI's routine analysis system are converted into PWV data by using the 12-hourly NWP data. While a front accompanying with heavy rainfall moved eastward across the Japanese Islands from 0900 local standard time (LST) September 1 to 0900 LST September 3, 1996, the GPS array successfully detected the temporal anomalies of GPS PWV moving along with the front, in which the internal errors are estimated to be less than 3 mm. The results reveal that GSI's GPS array can work as an all weather giant array sensor of PWV over the Japanese Islands. It is found, however, that the GPS shows systematically fewer PWV than NWP data in mountainous areas reaching about 10 mm. This bias results mainly from the fact that most GPS sites located at bottom of valley in mountainous areas. After removing the topographical effects, there still remain significant differences amounting to 2–4 mm associated with errors of GPS observations and/or NWP objective analyses data.