NOVELTY - Secondary battery comprises: a cathode; an anode; and a gel electrolyte, where the gel electrolyte includes an electrolytic solution and a polymer compound, and the electrolytic solution includes an unsaturated cyclic ester carbonate compound (I).    USE - The secondary battery is useful in battery pack, electric vehicle, electric power storage system, electric power tool, and in an electronic apparatus as an electric power supply source (all claimed), where the secondary battery includes mobile electronic apparatuses such as a video camcorder, a digital still camera, a mobile phone, a notebook personal computer, a cordless phone, a headphone stereo, a portable radio, a portable television and a personal digital assistant, a mobile lifestyle electric appliance such as an electric shaver, a memory device such as a backup electric power source and a memory card, and a medical electronic apparatus such as a pacemaker and a hearing aid, the electric vehicle includes an electric automobile, an electric motorcycle, and a hybrid automobile, the electric power storage system includes a home electric power server, and the electric power tool includes an electric drill and an electric saw. The secondary battery is useful for a machine, a device, an instrument, and a system.    ADVANTAGE - The secondary battery exhibits high energy density, and superior battery characteristics.    DETAILED DESCRIPTION - Secondary battery comprises: a cathode; an anode; and a gel electrolyte, where the gel electrolyte includes an electrolytic solution and a polymer compound, and the electrolytic solution includes an unsaturated cyclic ester carbonate compound of formula (I).    X = divalent group in which m number of =C=CR1-R2 and n number of =CR3R4 are bonded in any order;    R1-R4 = H or halo (preferably F, Cl, Br or I), monovalent hydrocarbyl, monovalent halogenated hydrocarbyl, monovalent oxygen-containing hydrocarbyl or monovalent halogenated oxygen-containing hydrocarbyl, where any two or more of R1-R4 are allowed to be bonded to one another;    m = greater than or equal to 1; and    n = greater than or equal to 0.    INDEPENDENT CLAIMS are included for:    (1) a battery pack comprising the secondary battery, a control section controlling a used state of the secondary battery, and a switch section switching the used state of the secondary battery according to an instruction of the control section;    (2) an electric vehicle comprising the secondary battery, a conversion section converting electric power supplied from the secondary battery into drive power, a drive section operating according to the drive power, and a control section controlling a used state of the secondary battery;    (3) an electric power storage system comprising the secondary battery, one or more electric devices supplied with electric power from the secondary battery, and the control section controlling the supplying of the electric power from the secondary battery to the one or more electric devices; and    (4) an electric power tool comprising the secondary battery, and a movable section being supplied with electric power from the secondary battery.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of a secondary battery.    Spirally wound electrode body (30)    Cathode lead (31)    Anode lead (32)    Film-like outer package member (40)    Adhesive film. (41)