To provide an improvement method of a soil material and a soil improvement material for allowing the transportation of a soft soil material of a high moisture content without using a cement-based solidification material.

A crushed bamboo material is mixed for 270[kg/cm3] or more to the soil material containing moisture equal to or more than the moisture content of a liquid limit. Thus, by mixing the crushed bamboo material in the soil material containing the moisture equal to or more than the moisture content of the liquid limit, it acts to be entangled with the soil material and gather it while absorbing the moisture of the soil material, and the soil material is improved to such strength that it can be put on a truck or the like and transported. Thus, without using the cement-based solidification material, the weak soil material of the high moisture content containing the moisture equal to or more than the moisture content of the liquid limit is improved to be put on a truck or the like and transported.