An all-solid-state lithium ion secondary battery containing a novel garnet-type oxide serving as a solid electrolyte. The garnet-type lithium ion-conducting oxide is one represented by the formula Li5+XLa3(ZrX, A2-X)O12, wherein A is at least one selected from the group consisting of Sc, Ti, V, Y, Nb, Hf, Ta, Al, Si, Ga, Ge, and Sn and X satisfies the inequality 1.4≦X<2, or is one obtained by substituting an element having an ionic radius different from that of Zr for Zr sites in an garnet-type lithium ion-conducting oxide represented by the formula Li7La3Zr2O12, wherein the normalized intensity of an X-ray diffraction (XRD) pattern with a diffraction peak, as normalized on the basis of the intensity of a diffraction peak, is 9.2 or more.