TECHNOLOGY FOCUS - ELECTRONICS - Preferred Components: The lithium ion conductive inorganic solid electrolyte particles are composite metal oxide comprising lithium lanthanum zirconium oxide compound (Li7La3Zr2O12), lithium lanthanum zirconium niobium oxide of formula Li7-yLa3Zr2-yNbyO12, Li7-yLa3-xYxZr2-yNbyO12, Li7-yLa3-xNdxZr2-yNbyO12, Li7-yLa3-xSmxZr2-yNbyO12, or Li7-yLa3-xGdxZr2-yNbyO12 or lithium lanthanum zirconium tantalum oxide of formula Li7-yLa3Zr2-yTayO12, Li7-yLa3-xYxZr2-yTayO12, or Li7-yLa3-xGdxZr2-yTayO12. The negative electrode active material comprises metal lithium or silicon.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The lithium ion conductive electrolyte solution comprises a lithium salt and an organic solvent capable of dissolving the lithium salt. The lithium salt comprises lithium hexafluorophosphate. The organic solvent comprises a mixed solvent of ethylene carbonate and propylene carbonate or mixed solvent of ethylene carbonate and diethyl carbonate. The porous body is formed on a porous substrate (3) in which a lithium ion conductive polymer gel electrolyte is held.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The organic polymer forming the porous body comprises a polyimide. The organic polymer forming the porous body comprises a mixture of styrene-butadiene rubber and polycarboxymethyl cellulose. The lithium ion conductive polymer gel electrolyte comprises a lithium ion conductive electrolyte solution and a polymer capable of internally absorbing the electrolyte solution. The polymer capable of internally absorbing the lithium ion conductive electrolyte solution comprises: polyacrylonitrile; and thermosetting polymer obtained by polymerization of polyethylene glycol diacrylate. EXAMPLE - Lithium hydroxide monohydrate was subjected to a dehydration treatment by heating for 6 hours at 350 degrees C in a vacuum atmosphere to obtain lithium hydroxide anhydride. Lanthanum oxide was subjected to a dehydration and decarboxylation treatment by heating for 24 hours at 950 degrees C in an air atmosphere. The lithium hydroxide anhydride, the dehydrated and decarboxylated lanthanum oxide, and zirconium oxide were weighed in a molar ratio of 7.7:3:2, and pulverized and mixed for 3 hours at a rotation speed of 360 rpm to prepare raw material mixture. The raw material mixture was charged into an aluminum crucible, and subjected to primary firing by holding for 6 hours at 900 degrees C in an air atmosphere to obtain a powdery primary fired product. The obtained primary fired product was then pulverized and mixed for 3 hours at a rotation speed of 360 rpm. Then, the mixture was charged into an aluminum crucible, and subjected to secondary firing by holding for 6 hours at 1050 degrees C in an air atmosphere. As a result, lithium ion conductive inorganic solid electrolyte particles formed from a composite metal oxide which is lithium-lanthanum oxide (Li7La3Zr2O12) and having a garnet structure were obtained. Composite metal oxide and polyimide varnish were weighed in a mass ratio of lithium ion conductive inorganic solid electrolyte particles:polyimide of 90:10, and stirred to obtain a mixture. The resultant mixture was defoamed, and mixed to obtain paste in which the lithium ion conductive inorganic solid electrolyte particles and the polyimide were uniformly dispersed. The paste was deposited by a casting method using a doctor blade onto a porous substrate formed from glass fiber non-woven cloth and then vacuum dried to give porous body. Lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate and propylene carbonate were mixed in a 1:1 volume ratio to prepare electrolyte solution in which lithium hexafluorophosphate concentration was 0.8 mol/l. Then, in glove box, 12 parts by mass polyacrylonitrile (150000 molecular weight) was mixed at 120 degrees C with 88 parts by mass of the electrolyte solution to obtain a sol-like liquid. Next, in a glove box, the sol-like liquid was impregnated under pressure into the laminate formed from the porous substrate and porous body. The impregnated laminate was left for 12 hours to allow the sol-like liquid to be cooled naturally and turned into a gel. Lithium ion conductive polymer gel formed in which the electrolyte solution was absorbed in the polymer gel formed from polyacrylonitrile. Thus, lithium ion conductive composite electrolyte was obtained in which the lithium ion conductive polymer gel electrolyte was held in the porous substrate and porous body.