A cylindrical Li7La3Zr2O12 specimen in contact with an Al2O3 crucible is sintered and the effect of Al diffusion on the microstructure, phase composition, and Li-ion conductivity of Li7La3Zr2O12 is investigated. The decrease in the Li-ion conductivity and Al concentration with increasing distance from the contact area with the Al2O3 crucible is confirmed using spatially resolved local-impedance spectroscopy and compositional analysis. The region close to the Al2O3 crucible shows abnormally large grains (size > 0.7 mm) within a matrix of fine grains (size: ∼10 μm), abundant intergranular liquid, the Li7La3Zr2O12 phase mostly in cubic form, and a high Li-ion conductivity (8.5 × 10−5 S cm−1). In contrast, the region far from the Al2O3 crucible exhibits uniform fine grains (size: ∼10 μm), scarce intergranular liquid, the tetragonal form of the Li7La3Zr2O12 phase, and a low Li-ion conductivity (1.1 × 10−6 S cm−1). The incorporation of Al into the Li7La3Zr2O12 lattice and the resulting increase in the number of Li-ion vacancies is suggested to be the main reason for the increase in the Li-ion conductivity. The origins of the abnormal grain growth induced by the diffusion of Al are discussed in relation to the atomic surface structures of Li7La3Zr2O12 and the presence of intergranular liquid phase.