PROBLEM TO BE SOLVED: To provide a bulk density adjustment material having versatility of soil treatment, and to provide a bulk density adjustment method.SOLUTION: A bulk density adjustment material includes a water-absorbing resin, a polymer compound other than the water-absorbing resin, and a bulk increasing material. When adding this to soil and mixing the mixture, the water-absorbing resin and the polymer compound absorb water in the soil to be swelled, combine and aggregate soil particles, reduces a bulk density of the soil, and further aggregate the soil to the degree of 5 mm. Hereby, handling of the treated soil becomes easy.SELECTED DRAWING: None