Disclosed is an air diffusion device including a base coupled to an air supply pipe and having an air discharge hole for discharge of air supplied from the air supply pipe, a perforated cover having a single-layered flat plate shape, and a securing mechanism to secure the perforated cover to the base. The perforated cover having through-holes is coupled to an upper surface of the base to cover the air discharge hole, defining an air guide chamber between the perforated cover and the base. The air diffusion device has ease in assembly/disassembly thereof because the perforated cover is assembled to the base via the securing mechanism separably coupled to the base. Further, through provision of a diaphragm cover to open or close an air discharge hole for discharge of air into the air guide chamber, it is possible to prevent wastewater, sludge, or impurities from entering the air supply pipe.