A header-equipped air diffusion device comprising: an air diffusion device and a header that are immersed in target treatment water, wherein the header includes an air storage portion that stores air, an air supply portion that feeds the air into the air storage portion , and an air feeding portion through which the air in the air storage portion is fed out, the air supply portion and the air feeding portion are provided in an upper portion of the air storage portion , a target treatment water-inlet port is formed in a lower portion of the air storage portion , the air feeding portion and the air diffusion device are connected to each other, and air which is fed from the header is diffused by the air diffusion device , and an air feeding port of the air feeding portion , the air feeding port being open in the air storage portion , is positioned higher than an air supply port of the air supply portion , the air supply port being open in the air storage portion , a difference in height (h1, h4) between the air feeding port of the air feeding portion and the air supply port of the air supply portion is 50 mm or greater, wherein the air diffusion device includes a horizontal pipe that is connected to the air feeding portion and extends in a horizontal direction, and a plurality of distribution portions that are provided to extend downward from the horizontal pipe with a gap therebetween in a longitudinal direction of the horizontal pipe and to distribute air from the horizontal pipe , an opening portion is formed on a side of the distribution portion opposite from the horizontal pipe , and a position of the air supply port of the air supply portion in a height direction is the same as a position of the opening portion of the distribution portion or is higher than the position of the opening portion . an air diffusion device and a header that are immersed in target treatment water, wherein the header includes an air storage portion that stores air, an air supply portion that feeds the air into the air storage portion , and an air feeding portion through which the air in the air storage portion is fed out, the air supply portion and the air feeding portion are provided in an upper portion of the air storage portion , a target treatment water-inlet port is formed in a lower portion of the air storage portion , the air feeding portion and the air diffusion device are connected to each other, and air which is fed from the header is diffused by the air diffusion device , and an air feeding port of the air feeding portion , the air feeding port being open in the air storage portion , is positioned higher than an air supply port of the air supply portion , the air supply port being open in the air storage portion , a difference in height (h1, h4) between the air feeding port of the air feeding portion and the air supply port of the air supply portion is 50 mm or greater, wherein the air diffusion device includes a horizontal pipe that is connected to the air feeding portion and extends in a horizontal direction, and a plurality of distribution portions that are provided to extend downward from the horizontal pipe with a gap therebetween in a longitudinal direction of the horizontal pipe and to distribute air from the horizontal pipe , an opening portion is formed on a side of the distribution portion opposite from the horizontal pipe , and a position of the air supply port of the air supply portion in a height direction is the same as a position of the opening portion of the distribution portion or is higher than the position of the opening portion . <EOS>