To use a monolithic silica body in chromatography with a HPLC column or a GC column and to simplify the use thereof as a separation medium, it is intended to provide a method of cladding a main body of a monolithic adsorbent or separating agent with glass so as to protect the outer surface, and to provide a separation medium prepared by the method. To this end, a monolithic silica body alone is formed by molding, and the molding is coated with a glass body; and then the glass body and the monolithic silica body are fused and integrated at the melting temperature of the glass body at an appropriate pressure. The surface of the resulting monolithic silica body clad with glass is strongly protected by the glass, and the homogeneity of the interior of the monolithic silica body is maintained, and thus uniform flow of a sample solution ensures analytical accuracy.