PURPOSE: To obtain an antioxidant which is water-soluble and excellent in heat stability by using a specific compound which has a radical-capturing property as an effective component.

CONSTITUTION: An antioxidant comprising a compound represented by the formula (wherein n is an integer of 4 to 8) or the salt thereof. The antioxidant inhibits the oxidative deterioration of auto-oxidative substances by heat or light, since the compound represented by the formula and its salt as the effective components have an excellent radical-capturing property. The compound and its salt can maintain their sufficient antioxidizing effect even at high temperatures, since they are excellent in heat stability. Further, they exhibit an antioxidizing effect even in an aqueous composition and do not impair the stability of the aqueous composition, since they have water-solubility. Particularly, they exhibit a significant effect in an aqueous polymer composition, since they do not impair the stability of the disperse system such as emulsion or dispersion.