In a membrane separation apparatus, a cleaning effect for membrane elements by scrubbing is improved. In the membrane separation apparatus 14 including a separation membrane 5 (membrane element) dipped in treated liquid in a treatment bath 15, and air diffusing means 2 for diffusing air bubbles 6 for cleaning the separation membrane 5, an air bubbles staying plate 3 is provided between the air diffusing means 2 and the separation membrane 5. The air bubbles staying plate 3 is configured to allow air bubbles 6 diffused from the air diffusing device 2 to stay, by which air bubbles 6 gather each other to form air bubbles 6 having large diameters. By causing air bubbles having large diameters to act on the separation membrane 5, air bubbles 6 can be allowed to act whole the separation membrane 5.