The present invention provides a medicine, comprising (a) an Otx2 protein or its partial peptide, or a salt thereof, or (b) a DNA or an RNA encoding an Otx2 protein or its partial peptide. The present medicine is useful as an agent for preventing, treating or suppressing progression of a retinal disease including retinal degeneration. In addition, the present medicine is useful, for example, as an agent for inducing differentiation from a retinal stem cell into a retinal photoreceptor cell, in the transplantation of a cell into the retina of patients suffering from retinal diseases.