An embodiment of the present invention relates to a lithium ion-conducting oxide or a lithium-ion secondary battery. The lithium ion-conducting oxide includes at least lithium, tantalum, phosphorus, M2, and oxygen as constituent elements, wherein M2 is at least one element selected from the group consisting of elements of the Group 14 and Al (provided that carbon is excluded), a ratio of number of atoms of each constituent element of lithium, tantalum, phosphorus, M2, and oxygen is 1:2:1−y:y:8, wherein y is more than 0 and less than 0.7, and the lithium ion-conducting oxide contains a monoclinic crystal.