Provided is a catalyst composition capable of preventing decrease in catalytic activity due to grain growth of noble metal under high temperature or under change in oxidation reduction or further for long term use, and of achieving excellent catalytic activity over a long time.
The catalyst composition containing a composite oxide represented by the following general formula (1):

        AO·x(B 2-y C y O 3-α )     (1)

(wherein A represents an element selected from monovalent elements, divalent elements and lanthanides; B represents a trivalent element; and C represents a noble metal; x represents an integer of 1 to 6; y represents an atomic ratio satisfying the following relation: 0