Effect on the ionic conductivity of B-site ion (M) substitution in (Li3x La2/3−x )1+y/2M y Ti1−y O3 (M=Al, Cr) has been investigated. It has been found that partial substitution of smaller Al3+ for Ti4+ is effective to enhance the ionic conductivity of Li3x La2/3−x TiO3. At 300 K, the maximum bulk conductivity of (1.58±0.01)×10−3 S cm−1 is observed from the composition of (Li0.39La0.54)1−y/2Al y Ti1−y O3 with y=0.02 (x=0.13), that is the highest yet reported for known perovskite solutions at room temperature. The conductivity enhancement is interpreted as being due to the substitution-induced bond-strength change rather than due to bottleneck size change for Li migration, TiO6-octahedron tilting or A-site cation ordering.