The present invention relates to a mixing ion conductive material for use in an electrochemical device such as a fuel cell or a sensor and more particularly to the super ion conductive material based on protons or oxide cations and also to a synthesizing method of the mixing ion conductive material.A mixing ion conductive oxide which has a proton conductivity and/or oxide cation conductivity and has a composition comprising 1 mol of barium oxide, 1-x mol of cerium oxide and x mol of gadolinium oxide; wherein 1>x>0.1. When x=0.2, the mixing ion conductive material shows a conductivity shown in FIG. 2. The oxide sintered body mentioned above has a density higher than 96.5% of the theoretical density and a grain size of 0.1 to 10 micron. The synthesizing method of the oxide sintered body is characterized by that the powders for use in the final heat treatment is crushed into a particle size less than 3 micron and is subjected to a vacuum drying process. Further, the sintering temperature is specified to a temperature between 1635.degree. C. and 1665.degree. C.