A mixed ionic conductor of the present invention includes a perovskite oxide of Ba, Ce, and In. The perovskite oxide is a crystalline compound expressed by Ba(Ce.sub.1-xIn.sub.x).sub.pO.sub.3, where x is 0.4 to 0.6 and p is 1 to 1.02. An electrochemical device of the present invention includes the mixed ionic conductor as a solid electrolyte. In the electrochemical device, electrons generated by an oxidation-reduction reaction are drawn in the thickness direction of the solid electrolyte. With this configuration, the present invention can provide a mixed ionic conductor that has high conductivity and high reliability, and an electrochemical device using the mixed ionic conductor.