NOVELTY - An oxygen surplus-type metal oxide specific complex of trivalent rare earth element, calcium, alkaline earth metal element, and oxygen tetracoordinate element. The metal oxide has capability of high speed/reversible oxygen diffusion at low-temperature region.    USE - For ceramics used for oxygen storage or oxygen selective film, and for oxidation reaction apparatus, oxygen enrichment apparatus, heating apparatus, cooling device and heat exchange apparatus (all claimed). Also used as ternary catalyst of exhaust gas purification apparatus of fuel cell used for motor vehicles.    ADVANTAGE - The metal oxide permits diffusion of large amount of oxygen at high speed in a reversible manner.    DETAILED DESCRIPTION - An oxygen surplus-type metal oxide is of formula: AjBkCmDnO7+~d, and has capability of high speed/reversible oxygen diffusion at low-temperature region.    A=trivalent rare earth metal and calcium;    B=alkaline earth metal element(s);    C,D=oxygen tetracoordinate element, and at least one is transition metal element;    j,k=more than 0;    m,n=0 or more;    j+k+m+n=6;and    delta=more than zero and 1.5 or less.    INDEPENDENT CLAIMS are included for the following:    (1) ceramics for oxygen storage or oxygen selective film, which contains the oxygen surplus-type metal oxide;    (2) storage, separation and concentration of oxygen, which involves carrying out storage, separation and concentration of oxygen using oxygen surplus-type metal oxide at low-temperature region. The molar quantity of oxygen in metal oxide is more than 0 and 21.4% or less;    (3) oxygen storage, separation and concentration apparatus;    (4) oxidation reaction apparatus, which carried out oxidation using the stored oxygen;    (5) oxygen enrichment apparatus, which carries out enrichment using the stored oxygen;    (6) heating apparatus, which carries out heating using warm temperature generated by storage, separation and/or concentration of oxygen;    (7) cooling device, which carries out cooling using cooling heat generated by storage, separation and/or concentration of oxygen; and    (8) heat exchange apparatus, which exchanges heat using warm temperature and/or cooling heat generated by storage, separation and/or concentration of oxygen.