Provided is a composition for preventing, ameliorating, or treating inflammatory diseases, the composition including, as active ingredients, lactic acid bacteria-derived extracellular vesicles isolated from culture fluid of lactic acid bacteria; and a method for diagnosing atopic dermatitis. The composition including the lactic acid bacteria-derived extracellular vesicles as active ingredients is expected to be usefully used in development of drugs, cosmetics, or health functional food for preventing, ameliorating, or treating inflammatory diseases such as atopic dermatitis, chronic rhinitis, chronic rhinosinusitis, asthma, chronic obstructive pulmonary disease, sepsis, etc. Also, the composition will be usefully used for diagnosis of atopic dermatitis by measuring the distribution of lactic acid bacteria-derived extracellular vesicles in a urine or blood sample.