Disclosed is a means that can be used for modifying a biologically or pharmacologically active substance from a form incapable of permeating the blood-brain barrier to a form capable of permeating the blood-brain barrier. Also disclosed is a modified substance obtained by using the means. The means is anti-human transferrin receptor antibody and the modified substance is a molecular conjugate of a biologically active protein or a pharmacologically active low-molecular-weight compound and anti-human transferrin receptor antibody.
