An air diffusion device is provided. The device includes a base coupled to an air supply pipe for supply of air. The base has an air discharge hole for discharge of air supplied from the air supply pipe. The device further includes a perforated cover in the form of a multi-layered stepped plate the height of which is reduced stepwise from the center to the edge. The perforated cover has a plurality of through-holes for discharge of air, and it is coupled to an upper surface of the base to cover the air discharge hole, thus defining an air guide chamber between the perforated cover and the base. The device further includes a securing mechanism configured to secure the perforated cover to the base.