An antioxidant material of the present invention contains flavonoid aglycon and vitamin C. The flavonoid aglycon is eriodictyol and/or diosmetin obtained by treatment of a raw material containing flavonoid glycoside derived from lemons, limes, or sudachis to form the aglycones thereof. It is preferred that this antioxidant material be produced through a step of mixing the flavonoid aglycon and the vitamin C after the treatment to form the aglycones. The treatment to form the aglycones is glycosidase treatment using a .beta.-glycosidase derived from a microorganism of the genus Aspergillus or Penicillium multicolor, or microbial fermentative treatment using a microorganism of the genus Aspergillus. An anti-deterioration agent and a food or beverage of the present invention contain the antioxidant material.