[en] Pre-excavation grouting of shafts and galleries had been conducted during the construction of Mizunami Underground Research Laboratory in the aspect of reducing water inflow into shafts and galleries. After excavating GL.-500m gallery, a post-excavation grouting was performed on section of the pre-excavation grouting area under high water pressure condition (maximum water pressure: 4MPa). The post-excavation grouting experiment was performed outside of the pre-excavation grouting zone with designs, applying colloidal silica grouting material and complex dynamic grouting. It was estimated that the inflow after post-excavation grouting was reduced by 1/100 of the case that pre- and post-excavation grouting were not performed. In addition, a new theoretical formulation considering relationship between reduction of hydraulic conductivity and the grouting zone was derived. This paper states the theoretical evaluation is very convenient and useful for grouting design and estimate of water inflow on these results. (author)