A series of Li₂O-La₂O₃-SiO₂ sinters were prepared with the various ratios of the associated elements, Li: La: Si = x: (10-x): 6, and their ionic conductivities were examined. All the sinters were mainly composed of the apatite-type Li_xLa_10-x(SiO₄)₆O_3-x crystalline phases but sub-phases were produced in some cases. The lattice parameters of the main phase decreased with an increase in the Li-content, x, up to x = 2.0 and became constant above x = 2.0, indicating that La 4f-site was partially replaced by Li up to the maximum of x = 2.0. The conductivity of the sinters gradually increased with an increase in x to 1.5, but it suddenly dropped down at x = 2.0 and increased again by a further increase in x. The XRD and ²⁹Si-NMR results suggested that the first conductivity increase was due to the decrease in the content of the La₂SiO₅ sub-phase with the Li replacement. Furthermore, the ⁷Li-NMR measurements gave evidence that lithium silicate sub-phase was formed at above x = 2 in the grain-boundary of the crystalline phase as a high Li-ionic conducting phase. The Nernst's EMF response of the O₂ and CO₂ concentration cells suggested that the main ionic carriers were oxide ion at x < 2.0 and lithium ion at x ≥ 2.