A garnet-type ion-conducting oxide configured to inhibit lithium carbonate formation on the surface of crystal particles thereof, and a method for producing an oxide electrolyte sintered body using the garnet-type ion-conducting oxide. The garnet-type ion-conducting oxide represented by a general formula (Lix-3y-z, Ey, Hz)LαMβOγ (where E is at least one kind of element selected from the group consisting of Al, Ga, Fe and Si; L is at least one kind of element selected from an alkaline-earth metal and a lanthanoid element: M is at least one kind of element selected from a transition element which be six-coordinated with oxygen and typical elements in groups 12 to 15 of the periodic table; 3≤x−3y−z≤; 0≤y≤0.22; C≤z≤2.8; 2.5≤α≤3.5; 1.5≤≈≤2.5; and 11≤γ≤13), wherein a half-width of a diffraction peak which has a highest intensity and which is observed at a diffraction angle (2θ) in a range of from 29° to 32° as a result of X-ray diffraction measurement using CuKα radiation, is 0.164° or less.