Damping device in a microwave oven including an air duct on an electrical compartment side of a partition wall for guiding an air flow from a cooing fan to a cooking chamber, a damper rotatably hinged on the air duct for selectively blocking the air duct, a driving cam having a relative thickness difference between a thick portion and a thin portion, rotatable in contact with one end of the damper for pressing the one end of the damper to rotate the damper centered on the hinge owing to the thickness difference, and control means for controlling rotation of the driving cam, thereby providing a smooth damping operation, reducing a device cost, and preventing the microswitch from being pressed excessively.