this the air introduced into the electric fitting room cools down the electric fitting room 113 in an upper part of the microwave oven and the air flowing toward the air inlet and lighting window 112 c through a space under the air duct 102 guided by the guide wall 114 cools down the lamp thus the lamp 106 at the lamp bracket 104 fitted to a lower part of the air duct 102 is cooled down by the foregoing air flow adequately the downward air flow with reference to the air inlet and lighting window 112 c prevent the water vapor and the like in the cavity 112 from escaping through the air inlet and lighting window 112 c on the other hand the air introduced into the cavity 112 through the air inlet and lighting window carries away the water vapor in the cavity 112 through the air outlet 112 b the f 1 f 2 and f 3 in fig7 illustrate an air flow in the ventilation structure of a microwave oven in accordance with a second preferred embodiment of the present invention wherein the air passed through the air inlet and lighting window 112 c flows as represented by f 1 guided by the guide wall flows as represented by f 2 in the cavity 112 and flows as represented by f 3 after the air escapes through the air outlet 112 b in the meantime the guide wall 114 may be formed as a unit with the air duct 102 or as a unit with the upper surface of the cavity 112 or an individual piece the lamp bracket 104 over the air inlet and lighting window 112 c may also be formed as a unit with the air duct 102 the.