Provided is a composition for preventing or treating IL-17-related diseases, more specifically, autoimmune diseases and inflammatory diseases induced by an increase of IL-17. The IL-.17 production inhibitory composition is characterized by containing mesenchymal stem cells originated from an adipose tissue as an active ingredient. This IL-17 production inhibitory composition is efficacious in preventing or treating diseases caused by the IL-17 production. The said diseases are selected from the group consisting of Kawasaki disease, microscopic polyangitis, adult-onset Still's disease, Stevens-Johnson syndrome and toxic epidermal necrolysis.
