A ceria-zirconia-based composite oxide which has a crystal phase of the composite oxide of a single solid-solution phase even after exposure to a high temperature over a long time and has a small change in mode pore diameter and in pore volume before and after a high temperature durability test is provided. This is realized by a ceria-zirconia-based composite oxide having a chemical composition, by mass ratio, of zirconia: 30% to 80%, a total of oxides of one or more elements selected from yttrium and rare earth elements having atomic number 57 to 71 (except cerium and promethium): 0% to 20%, and a balance of ceria and unavoidable impurities, in which ceria-zirconia-based composite oxide, the composite oxide is deemed to be a single solid-solution phase in an X-ray diffraction pattern after a durability test which heats the oxide in the atmosphere at a temperature condition of 1100° C. for 5 hours and has a ratio (b/a) of mode pore diameter (b) of a pore distribution after a durability test which heats the oxide in the atmosphere at a temperature condition of 1100° C. for 5 hours to the mode pore diameter (a) before the durability test of 1.0≤b/a≤2.0 and/or has a ratio (d/c) of pore volume (d) after a durability test which heats the oxide in the atmosphere at a temperature condition of 1100° C. for 5 hours to the pore volume (c) before the durability test of 0.20≤d/c≤1.00.