Protonic ceramic fuel cells (PCFCs) are expected to be a highly efficient next-generation fuel cell because the fuel is not diluted by the generated water vapor during power generation. In the past research, we have found that Yb-doped BaZrO₃ (BZYb) is promising in terms of the durability against CO₂, proton conductivity, and less reactivity with NiO used for the fuel electrode. We are currently developing improvement of power generation efficiency and durability of PCFCs using BZYb electrolyte. In terms of the durability of PCFCs, there is concern about degradation due to diffusion of transition metals such as Co and Ni used in the air and fuel electrodes. In this study, we evaluated the durability of PCFCs using BZYb electrolyte focusing on the diffusion of transition metals derived from electrodes. We also examined the effect of a buffer layer on the air electrode side. In this study, fuel electrode supported cells with BaZr_0.8Yb_0.2O_3-δ(BZYb20) electrolyte and NiO-BZYb20 fuel electrode was used. The fuel electrode half cells were fabricated by integrally firing process of the laminated green sheets of the electrolyte and fuel electrode to densify the electrolyte. For air electrode, BaZr_0.375Yb_0.125Co_0.50O_3-δ was screen printed on the fuel electrode half cell and fired at 900°C. For the cell with buffer layer between the air electrode and the electrolyte, BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_{3- δ} was spin-coated on the half-cell and then fired at 1300°C. The air electrode was then prepared in the same way. Durability tests were conducted at constant-current mode at 600°C, and the elemental diffusion state of the cells before/after the test was observed by cross-sectional EPMA. During 1000h power generation, the increase of ohmic resistance and polarization resistance was observed, and the increase in resistance was found to be smaller in cells with buffer layer. The results of cross-sectional EPMA analysis after the power generation test showed differences in the segregation state of Co and Ni in the electrolyte, with a tendency for segregation to be suppressed in the presence of a buffer layer. Acknowledgement: This work is partially based on results obtained from a project (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.