A cooking chamber assembly for a microwave oven, comprising: a case having a cooking chamber formed therein and an opening in a front part; an air duct directly fixed to an inside of an upper part of the case by one or more hooks and having a bottom surface forming a ceiling surface of the cooking chamber and providing an air circulating passage; wherein the air duct includes a panel forming the ceiling surface of the cooking chamber, walls around the panel, a plurality of apertures (625,635) for passing of air, and one or more elastic hooks each extended upward from the wall, wherein the air duct further includes a flange extended horizontally to forward from a top of the front wall, characterized in that the case includes a plurality of inserting holes in an upper part for inserting and fastening the hook(s), and the hook(s) include(s): an elastic body extended upward from the wall, and a wedge formed head having a predetennined gap between a bottom surface of the head and a top surface of the flange, wherein the gap is the same, or is smaller than a thickness of the upper part in which the inserting holes are formed, wherein a top of the head of the hook is sloped so that the head is pushed backward and an upper part of the elastic body is elastically pushed backward when the air duct is lifted until the respective hook is inserted in the respective inserting hole and the upper part of the elastic body and the head move forward by an elastic restoring force when the head is fully inserted in the inserting hole, and wherein the upper part in which the inserting hole is formed is fastened between the bottom surface of the head and the top surface of the flange. a case having a cooking chamber formed therein and an opening in a front part; an air duct directly fixed to an inside of an upper part of the case by one or more hooks and having a bottom surface forming a ceiling surface of the cooking chamber and providing an air circulating passage; wherein the air duct includes a panel forming the ceiling surface of the cooking chamber, walls around the panel, a plurality of apertures (625,635) for passing of air, and one or more elastic hooks each extended upward from the wall, wherein the air duct further includes a flange extended horizontally to forward from a top of the front wall, characterized in that the case includes a plurality of inserting holes in an upper part for inserting and fastening the hook(s), and the hook(s) include(s): an elastic body extended upward from the wall, and a wedge formed head having a predetennined gap between a bottom surface of the head and a top surface of the flange, wherein the gap is the same, or is smaller than a thickness of the upper part in which the inserting holes are formed, wherein a top of the head of the hook is sloped so that the head is pushed backward and an upper part of the elastic body is elastically pushed backward when the air duct is lifted until the respective hook is inserted in the respective inserting hole and the upper part of the elastic body and the head move forward by an elastic restoring force when the head is fully inserted in the inserting hole, and wherein the upper part in which the inserting hole is formed is fastened between the bottom surface of the head and the top surface of the flange. an elastic body extended upward from the wall, and a wedge formed head having a predetennined gap between a bottom surface of the head and a top surface of the flange, wherein the gap is the same, or is smaller than a thickness of the upper part in which the inserting holes are formed, wherein a top of the head of the hook is sloped so that the head is pushed backward and an upper part of the elastic body is elastically pushed backward when the air duct is lifted until the respective hook is inserted in the respective inserting hole and the upper part of the elastic body and the head move forward by an elastic restoring force when the head is fully inserted in the inserting hole, and wherein the upper part in which the inserting hole is formed is fastened between the bottom surface of the head and the top surface of the flange.