Abstract: The present invention relates to a coagulant for soil and/or sand and method for preventing washout using the same. The coagulant comprises approximately 10 parts by weight of carboxymethylcellulose, approximately 10 parts by weight of calcium chloride, approximately 10 parts by weight of sodium metasilicate powder, 0.3 to 100 parts by weight of silicon dioxide and 1 to 300 parts by weight of calcium carbonate. The method for preventing washout comprises the steps of scattering the coagulant over the surface of the soil and/or sand in a manner such that 360 to 1000 g of the coagulant is applied to a volume of surface area of 1 m.sup.2 and a depth of 10 cm, and excavating the soil and/or sand to a depth of 5 to 20 cm and mixing the coagulant and the soil and/or sand while adjusting water content of the soil and/or sand in the range of 30 to 45%. In the method, a part of --OH of cellulose in carboxymethylcellulose is denatured into --OCH.sub.3 which is soluble in water and highly adhesive.