The antioxidant according to the present invention contains a polyurethane compound represented by the following formula (1): (wherein Y1 is a residue of a diol, and plural Y1s may be the same or different, R1 is a residue of an aromatic diisocyanate, and plural R1s may be the same or different, X1 is a residue of a hindered phenol compound having at least k hindered phenolic hydroxyl groups, and plural X1s may be the same or different, m is an integer of 1 or more, n is an integer of 1 or more, and k is an integer of 2 or more).