An object of the present invention is to provide a pharmaceutical preparation that can promote the proliferation of oligodendrocyte precursor cells to effectively prevent or treat a demyelinating disease. FGF21 acts to promote the proliferation of oligodendrocyte precursor cells, and thus, the administration of FGF21, DNA encoding FGF21, an FGF21 production-promoting substance, and/or FGF21-producing cells is effective for preventing or treating a demyelinating disease.