The present invention relates to an antioxidant composition containing an extracellular polysaccharide produced using Ceriporia lacerata, or a mycelium culture of Ceriporia lacerata comprising the same, or a dry powder or an extract thereof as an active ingredient. The composition can be used as an antioxidant for preventing or treating various diseases such as brain diseases such as stroke, Parkinson's disease, etc., heart diseases, ischemia, arteriosclerosis, skin damage, inflammation, rheumatism, autoimmune diseases, etc. including cancers as well as aging, or as a health functional food, a cosmetic material or a feed composition having an antioxidant effect.