We estimated precipitable water vapor (PWV) with a high time resolution using Global Positioning System (GPS) measurements taken at Shigaraki, Japan, in 1995, and Yamagawa, Japan, in 1996. We have compared PWV determined from GPS data with radiosonde, radiometer and ceilometer data. Comparison between the GPS and radiosonde data shows that the GPS results, with a high time resolution of as little as 6 min. correctly provided the absolute PWV with an error of about 8 mm related to site coordinate errors. In a comparison between the GPS and radiometer (in the zenith direction) data, the GPS 6-minute-derived PWV agree in their perturbations with radiometer results. We also compared these good time resolution GPS results obtained at three receivers located within a distance of several hundred meters of each other. As a result, these PWV were in agreement within an absolute accuracy of about 4 mm. Finally, we compared the GPS results with the zenith delay caused by water vapor which was assumed to be contained in the lowest cloud layer, whose bottom height and thickness were determined with a ceilometer. We found good agreement of relative zenith delay perturbations between the GPS (estimated every 1 min.) and ceilometer results, especially in the case of a cloud bottom height of as low as 100 m within RMS of 1.0 mm. From these results, GPS has the potential to detect small changes of water vapor quantity.