[...] oxygen

PROBLEM TO BE SOLVED : To keep high oxygen storage capacity even in severe condition by employing an oxygen storaging cerium based compounded oxide having a specified formula for efficiently extracting the function of a catalyst to remove NOX, CO, etc., contained in an exhaust gas of an internal combustion engine of an automobile and as such. SOLUTION : As a cerium based compounded oxide whose specific surface area tends to decrease due to growth of cerium oxide at the time when the oxide is exposed to high temperature, an oxide having a prescribed formula is used (wherein R stands for rare earth metals, (z) for the oxygen deficient amount, and 0.2<=x+y<=0.9, 0.1<=x<=0.8, 0.05<=y<=0.3). In this cerium based compounded oxide, a part of cerium of cerium oxide crystal is preferably substituted with zirconium and rare earth metals and forms solid-solution, so that zirconium inhibits the mass transfer of cerium and suppresses grain growth of the cerium-based compounded oxide. Moreover, the compounded oxide is preferably used while bearing a noble metal.