To prepare mud mortar with a uniform strength even when a site for assuring a pit for mixing excavated sediments with different soil qualities is absent.

The pit 1 of sufficient volume is constructed at a site. Lumps of clay and silt are crushed and peptized, and then separated into fine grains and coarse grains. The fine grains and water are mixed with each other into slurry and the slurry is filled and stored in the pit 1. The coarse grains are stored in the form of soil grains. The slurry fine grains and coarse grains are mixed with each other at an appropriate ratio to adjust the density of the slurry. In addition, cement is added to the slurry and the mixture is agitated and mixed to prepare mud mortar. The mud mortar is used as back-filling material and filler material.