A membrane air diffuser in which air hole parts are opened by the expansion of an air diffusion membrane (3) in a mountain shape when viewed in a longitudinal direction (A) by the pressure of air supplied to the air diffusion membrane (3) when air is diffused, wherein a plurality of first air hole parts (8) and a plurality of second air hole parts (9) are formed while being arranged in the longitudinal direction (A) and the lateral direction (B) of the air diffusion membrane (3), the first air hole parts (8) are each a hole that is longer in the longitudinal direction (A) of the air diffusion membrane (3), the second air hole parts (9) are each a hole that is longer in the direction inclined with respect to the longitudinal direction (A) of the air diffusion membrane (3), and the second air hole parts (9) are each located between the first air hole parts (8); adjacent to each other in the longitudinal direction (A) and/or the lateral direction (B).