A plastic gel grouting material, which comprises a silica type non-hardening powdery material and water or a silica type non-hardening powdery material, a calcium type hardening developing material and water as active ingredients, is statically pressure-injected into a soft, loose or weak ground to thereby form masses derived from the grouting material per se in the ground, and concurrently therewith, to compact the surrounding earth and sand, thereby increasing density of the ground. A plastic gel grouting material is provided which loses its flowability with time or by dewatering to form masses derived from the grouting material per se in a ground and which comprises a silica type non-hardening powdery material and water or a silica type non-hardening powdery material, a calcium type hardening developing material and water as active ingredients.