Provided is a solid preparation showing improved stability of N-(4-(1-(2,6-difluorobenzyl)-5-((dimethylamino)methyl)-3-(6-methoxy-3-pyridazinyl)-2,4-dioxo-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidin-6-yl)phenyl)-N′；-methoxyurea and a salt thereof in the solid preparation, and a method of stabilizing the compound in a solid preparation.A tablet containing not less than 25 mass % of the compound； a solid preparation containing (1) the compound, and (2) a fat and oil-like substance having a low melting point, which is selected from polyethylene glycol, glycerol monostearate and triethyl citrate； a method of stabilizing the compound in a tablet, including adding not less than 25 mass % of the compound； and a method of stabilizing the compound, including adding a fat and oil-like substance having a low melting point, which is selected from polyethylene glycol, glycerol monostearate and triethyl citrate to a solid preparation containing the compound.
