An analysis of the diurnal variation of precipitable water and convective activity was conducted for sunny summer days around Mt. Tanigawa. Precipitable water and convective activity exhibited a pronounced diurnal variation, with dual maxima in the daytime (10-14 JST), and in the evening (18-22 JST), and these maxima of convective activity almost coincided with maxima of precipitable water. The mechanism responsible for the diurnal variation with dual maxima of precipitable water and convective activity, was discussed using GPS network data, and the findings of Iwasaki and Miki (2001 and 2002). The process of diurnal variation with dual maxima of precipitable water, around Mt. Tanigawa can be divided into 4 periods. 1) Late night to early morning: Precipitable water decreases due to large scale subsidence associated with a Pacific High, and the large scale subsidence persists throughout analysis period even in the daytime; 2) Early morning to noon: As a valley-wind circulation develops, moisture transport from the semi-basin (valley) to the mountain also becomes active. Because the effect of the moisture transport exceeds the effect of the large scale subsidence, precipitabe water around Mt. Tanigawa begins to increase gradually; 3) Noon to around 15 JST: Because the atmosphere in the valley becomes dry due to the compensating downdraft of the valley-wind circulation, the moisture transport from the semi-basin to the mountain decreased. The effect of large scale subsidence exceeds the effect of the moisture transport, so that precipitable water decreases in the daytime; and, 4) Around 15 JST to late night: Precipitable water is recovered due to moisture convergence from the socalled “extended sea breeze,” and moisture advection from the mountains, located on the windward side of Mt. Tanigawa. An increase of precipitable water in the daytime, and the evening, must work to unstabilize atmosphere, and surface heating, due to solar radiation that enhances the instability in the daytime. These effects contribute to formation, and/or development, of Cb groups in the daytime and the evening around Mt. Tanigawa.