A cooking oven, wherein air in a cooking chamber is sucked into a blower on the outside of the cooking chamber and fed to an upper duct and a lateral duct, air current led into the upper duct is heated by an upper heater and re-circulated from an upper blowing port to the cooking chamber, a catalyst block heated by a catalyst heating heater is disposed in the upper duct to decompose lamp black and smell substances contained in the air current, catalyst paint is applied onto the inner wall surface of the upper duct to reinforce a performance for decomposing the lamp black and smell substances, both the lateral duct and the upper duct are formed in the same structure, the distributions of heating volumes of the upper heater and the lateral heater in the cross sections of the upper duct and the lateral duct are set so that larger heating volumes are generated on larger air volume sides according to the air volume distributions in the cross sections of the ducts, and the catalyst block is also disposed in the cross sections of the ducts eccentrically to the larger heater heating volume side.