To prevent the clogging of the air diffusion port of an air diffusion pipe and to disperse air bubbles blown out of the air diffusion port throughout a liquid.

A membrane separation apparatus is equipped with a membrane separation tank 1, a filter membrane unit 3 consisting of a large number of vertical hollow yarn flat membrane modules 2 arranged mutually parallel so as to leave intervals between them and arranged in the membrane separation tank 1 and a plurality of air diffusion pipes 4 arranged under the filter membrane units 2 in the membrane separation tank 1. The air diffusion pipes 4 are arranged mutually parallel so as to provide intervals therebetween. A plurality of lateral air diffusion ports are formed to both side parts of the pipe wall of each of the air diffusion pipes 4 so as to provide intervals in the length direction of the air diffusion pipe. The air diffusion ports 6 of the adjacent pipes 4 are mutually opposed.