Compounds with perovskite structure which were candidates for high ionic conductivity were searched and synthesized on the basis of the knowledge of lanthanum lithium titanates and their ionic conductivity was investigated. The free volume for lithium ions to migrate, and the lithium and vacancy concentrations on the A-site play important roles for the ionic conductivity in the perovskite structure. Lanthanum lithium titanate substituted with 5 mol% Sr had a larger free volume and showed higher ionic conductivity of the bulk part (sigma = 1.5 X 10(-3)S cm-1 at 300 K) than the pure lanthanum lithium titanate.