THE PRESENT INVENTION PROVIDES A PHARMACEUTICAL COMPOSITION OR A SOLID PREPARATION CONTAINING A STABILIZED PHARMACEUTICALLY ACTIVE INGREDIENT AND A STABILIZING METHOD THEREOF. ACCORDING TO THE PRESENT INVENTION, A PHARMACEUTICAL COMPOSITION CAN BE STABILIZED BY CONTAINING A NONPEPTIDIC PHARMACEUTICALLY ACTIVE INGREDIENT HAVING A PRIMARY OR SECONDARY AMINO GROUP, AN EXCIPIENT AND AN ACIDIC COMPOUND. IN ADDITION, A SOLID PREPARATION CONTAINING A PHARMACEUTICALLY ACTIVE INGREDIENT, TITANIUM OXIDE, A PLASTICIZER AND A CHAIN ORGANIC ACID CAN ENHANCE THE STABILITY OF THE PHARMACEUTICALLY ACTIVE INGREDIENT DURING LIGHT IRRADIATION.