The electrochemical properties of the proton-conducting ceramic fuel cells (PCFCs) with BaZr0.1Ce0.7Y0.1X0.1O3-δ (BZCYX, X = Ga, Sc, In, Yb, or Gd) electrolytes have been investigated. BZCYX materials were found to have various partial conductivities of charge-carriers such as ion, hole, and electron. The electrochemical properties exhibited strong dependences on operation conditions. When ASR and external current density were fixed at 0.4 Ω cm2 and 0.25 A cm-2, respectively, the electrical efficiency, η(X), was found to have the following sequential order: η(Sc) > η(In) > η(Ga) > η(Yb) > η(Gd). On the other hand, when ASR was not fixed but the thickness of the electrolyte was fixed at 25 μm, large variations appeared in the leakage current of the cells with the BZCYX electrolytes. The sequential order of the electrical efficiency with the fixed thickness was different from that with the fixed ASR as described in the above inequality expression, and depends on the operating temperature. The ratios of the leakage current with X = Yb or Gd were higher than those with X = Ga, Sc, or In. These high ratios were found to cause the serious drop in the electrical efficiency at an external current density of 0.25 A cm-2. We have successfully found out the candidates for the X element in BZCYX, by which high-efficient power generation would be expected.