The present invention provides a cerium oxide-zirconium oxide based composite oxide that has a large OSC at a low temperature and that has a suitable OSC, and a method for readily producing the composite oxide. Specifically, the present invention provides a cerium oxide-zirconium oxide based composite oxide comprising a mixture of (1) a cerium-zirconium composite oxide from a melting process and (2) cerium dioxide from a wet process, and a method for producing a cerium oxide-zirconium oxide based composite oxide, the method comprising dispersing a cerium-zirconium composite oxide from a melting process in a cerium-containing solution, neutralizing the resulting dispersion; and then performing a heat treatment.