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.
