A novel selenium-containing compound, a method of producing the selenium-containing compound, an analysis method using the selenium-containing compound, and use of the selenium-containing compound as an antioxidant, are disclosed. A novel selenium-containing compound shown by any of chemical formulas 1 to 4 is obtained by extracting a selenium-containing compound from a sample using an organic solvent or water, and purifying the selenium-containing compound. The novel selenium-containing compound may be used for a novel analysis method, or may be used as an antioxidant.