The connection between the gut microbiome and various diseases has been attracting attention. The gut-brain axis has long been acknowledged as a close relationship between the gut and central nervous system (CNS). Moreover, the gut is closely related to the immune system, and both the gut environment and the immune system are implicated in the pathogenesis of immune-mediated neurological diseases such as multiple sclerosis as well as neurodegenerative diseases such as Parkinson’s disease, both of which have been associated with dysbiosis. Although the exact mechanisms by which the gut environment regulates CNS inflammation remain unclear, metabolites or components produced by gut bacteria affect not only the immune system but also glial cells and contribute to disease pathology.