An overcharge suppressing agent adapted to react when the positive electrode potential becomes higher, to increase the internal resistance of a battery during overcharge in an lithium ion secondary battery in which a positive electrode capable of occluding and releasing lithium and a negative electrode capable of occluding and releasing lithium are formed by way of an electrolyte. The electrolyte contains a polymerizable compound represented by the chemical formula (1-1) or the chemical formula (1-2):
Z1-A Chemical formula (1-1)
Z1-X-A Chemical formula (1-2)
in which Z1 is a polymerizable functional group, X is a hydrocarbon group or an oxyalkylene group having 1 or more and 20 or less carbon atoms, and A is an aromatic functional group.