The high temperature cubic phase of Ba2 nr2O5 shows large ion conductivity. It is interestingto examine, if the cubic phase can be stabilized in the low temperature region (920 C) by making solid solution of another element. In the present study, we investigated the ion conductivity and the crystal structure of Ba2(In2-x.Mx)O5 system by substituting In site for element M such as Sc, Y, La, Ce, Nb, Ta etc. By substituting 3 mole % Nb for In, the transition temperature decreased by about 300 C. High temperature X-ray diffraction analysis shows the crystal structure changes from orthorhombic to cubic at this transition temperature. The effective elements which decreased the transition temperature were pentavalent or tetra valent elements such as Nb or Si ,Ce. The substitution In site for 20 mole percentage Nb stabilizes the cubic structure down to room temperature. Considering the transport number, the tetravalent element doping is very effective to stabilize the cubic phase of Ba2In2O5 without lose of excellent characteristic of pure Ba2ln2O5.