NOVELTY - A composite type oxide ion mixed conductor composition contains an oxygen ion conduction phase having specific rare earth dope cerium oxide, and electronic conduction phase having specific perovskite type complex oxide.    USE - For composite material for chemical reactor used for deoxygenation and film-type reactor for manufacture of synthesis gas (all claimed).    ADVANTAGE - The composite material using the composite type oxide ion mixed conductor composition has excellent stability and oxygen permeability, and is inexpensive. The composite material efficiently suppresses generation of carbonaceous precipitate during hydrocarbon modification reaction.    DETAILED DESCRIPTION - A composite type oxide ion mixed conductor composition contains an oxygen ion conduction phase having rare earth dope cerium oxide, and electronic conduction phase having perovskite type complex oxide. The molar ratio of rare earth dope cerium oxide with respect to perovskite type complex oxide is 0.1-20. The rare earth dope cerium oxide is given by formula: CeaA1-aO~c. The perovskite type complex oxide is given by formula: SrbX1-bCocFedZeO~d.    a=0.6-0.95;    A=rare earth element(s);    gamma=value selected such that electric charge of compositional formula is neutral;    X=yttrium, calcium, barium and/or lanthanoids;    Z=niobium and/or tantalum, titanium, aluminum and/or zirconium;    b=0.6-1.0;    c,d=greater than or equal to0 and less than 1.0;and    c+d+e=0.98-1.02.    The total number of moles of titanium and zirconium is 0-20% with respect to total number of moles of Z element. INDEPENDENT CLAIMS are included for the following:    (1) composite material having compact substance continuous layer formed on a porous support which contains magnesia. The support has thickness of 0.5-3 mm and porosity of 30-79%. The compact substance continuous layer contains the composite type oxide ion mixed conductor composition, and has thickness of 0.001-1 mm; and    (2) chemical reactor using the composite type oxide ion mixed conductor composition or composite material for segregation or chemical reaction region.