An object is to provide a drug which is useful in treating a nervous system disease. A drug containing vitamin B12 as an active ingredient according to the present invention has an M2 macrophage/microglia induction promoting effect, an M1 macrophage/microglia induction inhibiting effect, a nerve regeneration promoting effect, and the like and is very useful as a therapeutic agent for a nervous system disease, and particularly useful as a therapeutic agent for a central nervous system disease such as cerebral infarction, dementia, or spinal cord injury.