Header-equipped air diffusion devices includes, in the header, an air storage unit, on its lower end including inlet(s) for water to be treated, and air supply part(s) and air sending part(s) on the air storage unit upper section. The air diffusion device's air sending part and horizontal tube are connected, air sent from the header being diffused by the air diffusion device, and air sending in the air storage unit is above the air supply part's air supply port. The air storage portion's partition portion, with a 50+mm height, partitions the upper portion into an air supply and an air feeding portion side. The partition portion forms a cylindrical portion and an upper plate portion and the air storage portion's trunk portion serves as part of the air supply portion, and an opening end on a lower end side of the partition portion serves as the air supply port.