To provide an electrolyte additive capable of further improving the potential stability with respect to repeated charging and discharging.SOLUTION: An electrolyte additive, which is a mixture of an amide compound represented by CH2=CH-C(=O)-NR1R2 (R1 and R2 are the same or different to indicate hydrogen atoms or hydrocarbon groups that may be substituted) and a naphthalene derivative having 1 to 3 (same or different) hydroxy groups or alkyl groups as substituents, is used so as to be added to a non-aqueous electrolytic solution of a lithium ion secondary battery.SELECTED DRAWING: None