When we know an intrinsic diffusion coefficient of Li-ion (D) in an ionic conductor, we could estimate a number of mobile Li ions, i.e. carrier density (n), from ionic conductivity. As a result, we will judge which parameter is more important to determine ionic conductivity, D or n. Such information gives a significant insight for developing the ionic conductors with higher conductivity. Muon-spin spectroscopy is found to provide information on the intrinsic diffusion properties in many battery materials with good spacial and depth resolution. Thus, we wish to study the diffusive nature of a novel Li ion super-ionic conductor, Li10GeP2S12 and related compounds, which exhibit the highest ionic conductivity in solid electrolytes. The proposed muSR experiment will clarify the mechanism behind the super-ionic conducting behavior through the variation in D or n with the compositional change.