Problem to be solved.To provide novel antioxidant compositions containing a natural product or a primary processed product thereof as an active ingredient. Means for solving the problem. Provided is an antioxidant composition containing a fermented milk product of Lactobacillus kefiri or a processed product thereof as an active ingredient. This antioxidant composition can be used to reduce oxidative stress. Further, the Lactobacillus kefiri is preferably Lactobacillus Kefiri P-IF (accession number FERM BP-10896).