Co-catalyst for purifying exhaust gas

A co-catalyst for purifying an exhaust gas which can be used for a loner period of time as an actual catalyst by using the cerium oxide in the conventional co-catalyst for purifying the exhaust gas as a cerium-containing complex oxide for elevating the resistance to heat and suppressing the performance reduction due to thermal deterioration and by making a specific surface area and an oxygen storage capacity over specified values. A co-catalyst for purifying an exhaust gas of the present invention includes a composite oxide including (a) cerium; and (b) at least one element selected from the group consisting of zirconium, yttrium, strontium, barium and a rare-earth element supported on a particulate aluminum oxide support; a specific surface area of the co-catalyst after sintering being not less than 40 m2/g; an oxygen storage capacity at 400° C. being not less than 10 μmols/g and an oxygen storage capacity at 700° C. being not less than 100 μmols/g.