The present invention relates to a diffuser which is used in a reactor of sewage and waste water treatment equipment, lakes and marshes, rivers, fish farms and the like, and disperses air supplied from a blower into microbubbles in the water, thereby increasing dissolved oxygen. The present invention provides a membrane diffuser comprising: a flexible membrane comprising a plurality of aeration holes capable of generating microbubbles in the water; a body comprising an air supply hole and an air outlet hole for being supplied with air from the blower to supply the same to the membrane; and a fixing frame for fixing the membrane, wherein a protective net body is formed on the upper surface of the membrane, a stopper or a protrusion for preventing counterflow is formed to prevent the counter-inflow of a body of water into the diffuser and an air supply pipe, and a first body and a second body of the body and the fixing frame are molded with a synthetic resin to be firmly bonded as an integral body. The sealing property of a joined portion is excellent, the over-expansion and breakage of the membrane are prevented, and the counter-inflow of a body of water into the diffuser and an air pipe is blocked, resulting in convenient maintenance and a stable maintenance of a high oxygen transfer rate.