Characteristics of the water-vapor field in relation to thunderstorms on summer days over the Kanto district in Japan were studied, using precipitable water-vapor (PWV) derived from GPS during 2001‒2005. PWV averaged on the active thunderstorm days showed distinct diurnal variation. In general, hourly data showed that PWV maximum appeared 1~ 2 hours prior to the maximum thunderstorm activity (precipitation or cloud to ground stroke). The increase of PWV in the mountainous region of northern Kanto coincided well with the increase of low-level wind toward the mountains, which was observed by Wind profiler Network and Data Acquisition System (WINDAS). Using the 5 minute data, we further examined the time lag between the PWV and cloud to ground (CG) stroke related to individual thunderstorms in detail. The PWV maxima preceded that of CG stroke by 15~ 30 minutes, for about 40% of the thunderstorms. In many cases, both PWV and its increment in 30 minutes showed large values within one hour before the CG stroke occurrence. This suggested that GPS derived PWV appears to reflect well the local variations associated with a thunderstorm.