The present invention relates to a composition for treating inflammatory brain disease comprising stem-cell-derived exosome as an active ingredient. The stem-cell-derived exosome according to the present invention can advantageously be used in the treatment of inflammatory brain disease, since the exosome has outstanding therapeutic effects including suppressing cell death due to inflammation in nerve cells, and, in an inflammatory-brain-disease animal model, increasing survival rates, and markedly reducing brain damage and the inflammatory cytokine count due to inflammatory reactions such as oedema.