Complex oxide, method for producing same and exhaust gas purifying catalyst

Disclosed are a composite oxide which is capable of maintaining a large specific surface area even used in a high temperature environment, and which has excellent heat resistance and reducibility, as well as a method for producing the composite oxide and a catalyst for exhaust gas purification employing the composite oxide. The composite oxide contains cerium and at least one of rare earth metal elements other than cerium and including yttrium, at a mass ratio of 85 : 15 to 99 : 1 in terms oxides, and further containing silicon at more than 0 parts by mass and not more than 20 parts by mass in terms of SiO2 with respect to 100 parts by mass of the total of the cerium and the at least one of rare earth metal elements other than cerium and including yttrium, wherein the composite oxide has a specific surface area of not less than 40 m2/g as measured by the BET method after calcination at 900° C. for 5 hours, and a reducibility of not lower than 30% as calculated from measurement of temperature-programmed reduction from 50° C. to 900° C. after calcination at 1000° C. for 5 hours, and is particularly suitable for a co-catalyst for a catalyst for exhaust gas purification.