As a preparation method of proton conducting BaCe_1−xY_xO_3−δ, solid state reaction method and liquid phase mixing one from same raw materials have been compared. By solid state reaction of BaCO₃, CeO₂ and Y₂O₃, segregation of Y₂O₃ was observed in the specimens with x larger than 0.15. Employing liquid phase mixing method, BaCe_1–xY_xO_3–δ single phase of 0.00 ≤ x ≤ 0.25 with homogeneous Y distribution, has been successfully prepared. It was revealed that crystal system of BaCe_1−xY_xO_3−δ was orthorhombic distorted perovskite and monoclinic distorted one for 0.00 ≤ x ≤ 0.10 and 0.15 ≤ x ≤ 0.25, respectively. The abrupt increase of volume due to the structural phase transition was observed at x between 0.10 and 0.15.