The purpose of this invention is to provide a drug which is useful in the treatment of nervous system disease. This drug, which contains vitamin B12 as an active ingredient, has an action of promoting the induction of M2 macrophages/microglia, an action of suppressing the induction of M1 macrophages/microglia, and an action of promoting nerve regeneration, and is extremely effective as a drug for the treatment of nervous system diseases, particularly central nervous system diseases such as cerebral infarction, dementia and spinal cord injury.