Using various kinds of trivalent dopants, perovskite-type oxide solid solutions composed of BaCe1-MxO3-alpha were synthesized and their conduction properties at high temperature were studied electrochemically. These oxides showed mixed conduction of protons and oxide ions under fuel cell condition. The transference number of proton decreased and that of oxide ion increased as the radius of dopant cation M3+ grows large. The reason for it is discussed in relation to lattice parameters of orthorhombic crystal and easiness of migration of oxide ion through it.