Provided are: a solid preparation which improves the stability of N-(4-(1-(2,6-difluorobenzyl)-5-((dimethyl amino)methyl)-3-(6-methoxy-3-pyridazinyl)-2,4-dioxo-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidine-6-yl)phenyl)-N'-methoxyurea and a salt thereof which are present in the solid preparation； and a method for stabilizing the compound in the solid preparation. A tablet containing the compound in an amount equal to at least 25 mass%； a solid preparation containing (1) the compound, and (2) a low-melting-point oil/fat-like substance selected from polyethylene glycol, glyceryl monostearate, and triethyl citrate； a method for stabilizing the compound present in the tablet and characterized by formulating the compound in an amount equal to at least 25 mass%； and a method for stabilizing the compound and characterized by formulating a low-melting-point oil/fat-like substance selected from polyethylene glycol, glyceryl monostearate, and triethyl citrate in a solid preparation containing the compound.
