NOVELTY - A gel electrolyte contains a lithium salt, a copolymer of vinyl monomers (I) or (II) and (IV), compound having phosphazene structure (V) and at least one compound (A) selected from cyclic disulfonic acid ester (VI), chain disulfonic acid ester (VII) and sultone compound (VIII).    USE - Gel electrolyte is used for lithium ion secondary battery (claimed). Uses include but are not limited to power supply for motor vehicles, mobile telephone, notebook personal computer, and large-sized electric power storage.    ADVANTAGE - The lithium ion secondary battery comprising gel electrolyte is highly safe to use and has favorable durability.    DETAILED DESCRIPTION - A gel electrolyte contains a lithium salt, a copolymer of vinyl monomers, compound having phosphazene structure and at least one compound selected from cyclic disulfonic acid ester of formula (VI), chain disulfonic acid ester and sultone compound of formula (VIII). The vinyl monomer is of formula: CH2=C(-R1)-C(=O)-O-CH2-R2 (I) or CH2=C(-H)-CH2-O-CH2-R2 (II) and CH2=C(-R4)-R5 (IV). The compound having phosphazene structure is of formula: -(-P(-X1)(-X2)=N-)n- (V). The chain disulfonic acid ester is of formula: R8-S(=O)2-C(-R9)(-R6)-S(=O)2-R7 (VII).    R1,R4=H or CH3;    R2=epoxy, tetrahydrofuranyl, octahydro isobenzofuranyl or group of formula (i);    R3=1-6C alkyl;    R5=-COOCH3, -COOC2H5, -COOC3H7, -COOC4H9, -COOCH2CH(CH3)2, -COO(CH2CH2O)mCH3, -COO(CH2CH2O)mC4H9, -COO(CH2CH2CH2O)mCH3, -COO(CH2CH(CH3)O)mC2H5, -OCOCH3, -OCOC2H5 or -CH2OC2H5;    m=1-3;    X1,X2=halo, alkyl, alkoxy, aryl, acyl, aryloxy, amino, alkylthio, arylthio, haloalkyl, haloalkoxy, haloaryl, haloacyl, haloaryloxy, haloamino, haloalkylthio or haloarylthio;    n=3-5;    Q=O, methylene group or single bond;    A1=carbonyl, sulfinyl or 1-5C alkylene, 1-5C perfluoroalkylene, 2-6C fluoroalkylene, 1-6C alkylene including ether bond, 2-6C fluoroalkylene including ether bond or 1-6C perfluoroalkylene including ether bond (all optionally substituted and branched);    A2=optionally branched (un)substituted alkylene;    R6,R9=H, halo, 1-5C perfluoroalkyl, -SO2X3, -SY1, -COZ, or 1-5C alkyl, 1-5C alkoxy or 1-5C fluoroalkyl (all optionally substituted);    X3,Y1=(un)substituted 1-5C alkyl;    R7,R8=1-5C alkyl, 1-5C alkoxy, 1-5C fluoroalkyl, 1-5C perfluoroalkyl, 1-5C fluoroalkoxy (all optionally substituted), 1-5C polyfluoroalkoxy, hydroxy, halo, -NX4X5 or -NY2CONY3Y4;    Y2-Y4,X4,X5,Z=H or (un)substituted 1-5C alkyl;    R10-R15=H, 1-12C alkyl, 3-6C cycloalkyl or 6-12C aryl;and    s=0-2.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of structure of lithium ion secondary battery element after winding.    Aluminum foil (2)    Positive electrode active material application portion (3)    Positive electrode electroconductive tab (6)    Copper foil (8)    Negative electrode active material application portion (9)