NOVELTY - Lithium ion conductive composite electrolyte (1) comprises a composite electrolyte in which a lithium ion conductive polymer gel electrolyte is held in a porous body (2) formed from lithium ion conductive inorganic solid electrolyte particles, where the porous body is formed from an organic polymer, and lithium ion conductive inorganic solid electrolyte particles that are formed from a composite metal oxide that is substituted lithium-lanthanum-zirconium metal oxide (I) and that has a garnet structure.    USE - The lithium ion conductive composite is useful in electrolyte lithium ion secondary battery (claimed).    ADVANTAGE - The lithium ion conductive composite electrolyte: prevents leakage of electrolyte solution, and that has excellent cycle performance; and provides the battery with excellent charge/discharge capacity even when using silicon for negative electrode.    DETAILED DESCRIPTION - Lithium ion conductive composite electrolyte (1) comprises a composite electrolyte in which a lithium ion conductive polymer gel electrolyte is held in a porous body (2) formed from lithium ion conductive inorganic solid electrolyte particles, where the porous body is formed from an organic polymer, and lithium ion conductive inorganic solid electrolyte particles that are formed from a composite metal oxide that is substituted lithium-lanthanum-zirconium metal oxide of formula Li7-yLa3-xAxZr2-yMyO12 (I) and that has a garnet structure.    x = 0-3;    y = 0-2;    A = Y, Nd, Sm or Gd; and    M = Nb or Ta.    The lithium ion conductive inorganic solid electrolyte particles and the organic polymer forming the porous body have a volume ratio of 54:46-91:9, preferably 81:19-91:9. An INDEPENDENT CLAIM is also included for a lithium ion secondary battery comprising a positive electrode that includes a positive electrode active material, negative electrode that includes negative electrode active material, and lithium ion conductive composite electrolyte that is provided between the positive electrode and the negative electrode and in which a lithium ion conductive polymer gel electrolyte is held in a porous body formed from lithium ion conductive inorganic solid electrolyte particles.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional diagram illustrating a structure of the lithium ion conductive composite electrolyte.    Lithium ion conductive composite electrolyte (1)    Porous body (2)    Porous substrate (3)