NOVELTY - An overcharge inhibitor comprises a polymerizable monomer having repeating unit containing ether or ester.    USE - In nonaqueous electrolyte solution which is useful in secondary battery (claimed). Also useful as nonaqueous electrolyte secondary battery for a power source of a mobile information equipment such as a cellular phone or a notebook, and a power source for various devices such as an electric vehicle or a hybrid electric car.    ADVANTAGE - The overcharge inhibitor has the high electrochemical stability in the battery, and can be used without impairing the battery performance; and a high polymerization velocity as compared to the monomer, and hence has quickly increased the internal resistance. The overcharge inhibitor is more excellent than the low-molecule weight compound such as cyclohexyl benzene. The overcharge inhibitor does not react in an operating voltage range of the battery, and is electropolymerized at the time of the overcharge, while increasing the internal resistance of the battery, thus shutting down the battery reaction. The overcharge inhibitor is superior to the thiophene polymer. The overcharge inhibitor increases the internal resistance of the battery in overcharging the battery to increase the overcharge voltage, which enables appropriate detection of the charged state of the battery. Thus, the overcharge inhibitor is effective in control of the battery. The overcharge inhibitor provides a lithium ion battery having an overcharge inhibiting function with excellent safety.    DETAILED DESCRIPTION - An overcharge inhibitor comprises a polymerizable monomer as a repeating unit, of -(X)a-(Y')b- (I).    X=one polymerizable monomer, and has a functional group that is electropolymerized at a potential of 4.3-5.5 V based on a lithium metal reference (preferably 8 groups of formulae as given in the specification e.g. group of formulae (ia), (ii), (iii) or (iv));    Y'=other polymerizable monomer, having ether bond or ester bond or X (preferably 8 groups of formulae as given in the specification e.g. group of formulae (ia), (ii), (iii) or (iv));    a= greater than or equal to 1;    b=0 to greater than or equal to 1;    R1 and R4=functional group having biphenyl, cyclohexyl phenyl or naphthyl;    R2 and R5=1-3C alkyl (in which a hydrogen atom of alkyl is optionally substituted by F);    R3=1-6C alkyl or aromatic group (in which hydrogen atom of alkyl or aromatic group is optionally substituted by F) or 1-6C alkyl or aromatic group via alkoxy group of formula -(R7-O)g;    R6=1-6C alkyl or aromatic ring (in which a hydrogen atom of alkyl or aromatic group is optionally substituted by F);    c and e= greater than or equal to 1;    d and f=0 to greater than or equal to 1;    R7=2-6C alkyl or phenylene (in which a hydrogen atom of alkyl or phenylene is optionally substituted by F);    g=1-10;    x1=1-10;    m1= greater than or equal to 1;    n1=0 to greater than or equal to 1;    R8=1-6C alkyl or aromatic ring (in which a hydrogen atom of alkyl or aromatic group is optionally substituted by F);    p1= greater than or equal to 1;    q1=0 to greater than or equal to 1;    R11=1-6C alkyl or aromatic ring (in which a hydrogen atom of alkyl or aromatic group is optionally substituted by F).    INDEPENDENT CLAIMS are included for the following:    (1) a nonaqueous electrolyte solution comprising: a nonaqueous solvent; an electrolyte salt; and the overcharge inhibitor; and    (2) a secondary battery comprising: a positive electrode; a negative electrode; a separator; and the nonaqueous electrolyte solution.