An object of the present invention is to provide an industrially suitable process for preparing an imidazolidin-2,4-dione compound which is safe, simple and easy to prepare the imidazolidin-2,4-dione compound with high yield, which is a useful compound, for example, as a material for decomposing a harmful halogenated aromatic hydrocarbon compound such as dioxin, etc., an electroless silver plating solution for electronic parts, or a diazo copying material, etc. This object can be solved by a process for preparing an imidazolidin-2,4-dione compound which comprises subjecting a 4,5-dihydroxy-2-imidazolidinone compound, (1) to dehydration reaction in the presence of an acid catalyst(s); or (2) to reaction at 100 to 300° C. Or, it can be solved by a process for preparing an imidazolidin-2,4-dione compound which comprises reacting a mixed solution of a urea compound, glyoxal, and a base at 20 to 300° C.Also, an object of the present invention is to provide a method for acquiring a 4,5-dihydroxy-2-imidazolidinone from an aqueous solution containing a 4,5-dihydroxy-2-imidazolidinone by a simple and easy method. This object can be solved by a process for acquiring a solid state 4,5-dihydroxy-2-imidazolidinone compound which comprises mixing an organic solvent with an aqueous solution containing a 4,5-dihydroxy-2-imidazolidinone compound, and subjecting the mixture to azeotropic distillation.