Cooking chamber assembly in a microwave oven for improved assembly and air tightness. A cooking chamber is formed inside of the case, and a tray is provided inside of the case to form a bottom of the cooking chamber. A unitary gasket has a groove in an inner edge for inserting on an edge of the tray, and a lip on an outer edge in contact with an inside surface of the case for enhancing air tightness. An air duct provided to a ceiling of the cooking chamber for guiding circulating air is attached to an inside of the case directly with hooks without fastening members, such as screws, or bolts.