The present invention provides a pharmaceutical composition for preventing or treating inflammatory diseases, containing, as an active ingredient, stem cells overexpressing SOD3. The present inventor has ascertained that mesenchymal stem cells (MSCs) overexpressing SOD3 have a stronger antioxidant activity and immune regulatory function than normal MSCs, and thus the MSCs overexpressing SOD3 can be an effective treatment agent for inflammatory diseases, autoimmune diseases, organ transplant rejection and the like.
