<p>An air diffuser (1) is provided with an aeration tube (11) disposed directly under a membrane module (2) and aeration tubes (12) disposed parallel to the aeration tube (11) in positions not directly below the membrane module. While a plurality of opening parts are formed along the axial direction of the tube (11) on the lower surface of the aeration tube (11) in the position directly below the membrane module (2), a plurality of aeration holes with a diameter smaller than the opening parts are formed so as to be disposed in a position, which is not directly below the membrane module and is higher than the opening parts and lower than the axis of the tube (11) along the axial direction and so as to sandwich a cross-section in that direction. While a plurality of opening parts are formed on the lower surface of the aeration tubes (12) along the axial direction of the tubes (12), a plurality of aeration holes with a diameter smaller than the opening parts are formed so as to be positioned higher than the opening parts and lower than the axis of the tubes (12) along the axial direction and so as to sandwich a cross-section in that direction.</p>
