Abstract: A method for preventing accumulation of sludge in a coal water mixture storage tank. The method involves:preparing a coal and water mixture storage tank having a circulation pump for taking a stored liquid out of the storage tank, a liquid conducting pipe vertically standing inside of the storage tank for conducting the stored liquid discharged from the circulation pump to a bottom of the storage tank, one or more nozzles for jetting the stored liquid out of the liquid conducting pipe toward a radial direction of the storage tank and provision for rotating the liquid conducting pipe having an axis at a center of the storage tank;determining a zone width r(m) of a stirring zone of one of the nozzles, using a formula of:r.ltoreq.Q/(140.times.A.sup.0.5)where, A is a sectional area (m.sup.2) of a nozzle opening and Q is a flow amount of the stored liquid per one nozzle (m.sup.