To provide a lithium ion conductive crystal body having a high density and a large length and an all-solid state lithium ion secondary battery containing the lithium ion conductive crystal body. A Li5La3Ta2O12 crystal body, which is one example of the lithium ion conductive crystal body, has a relative density of 99% or more, belongs to a cubic system, has a garnet-related type structure, and has a length of 2 cm or more. The Li5La3Ta2O12 crystal body is grown by a melting method employing a Li5La3Ta2O12 polycrystal body as a raw material. With the growing method, a Li5La3Ta2O12 crystal body having a relative density of 100% can also be obtained. In addition, the all-solid state lithium ion secondary battery has a positive electrode, a negative electrode, and a solid electrolyte, in which the solid electrolyte contains the lithium ion conductive crystal body.