Disclosed is a membrane separation device wherein the cleaning effect of a membrane element by scrubbing is improved. The membrane separation device (14) comprises: separation membranes (5) (membrane elements) which are immersed in a liquid to be processed that is inside a processing tank (15); an aeration means (2) which diffuses bubbles (6) for cleaning the separation membranes (5); and a bubble entrapment plate (3) provided between the aeration means (2) and the separation membranes (5). The bubble entrapment plate (3) gathers the bubbles (6) diffused from the aeration means (2) by entrapping the bubbles (6) and forms bubbles (6) of a large diameter. The cleaning effect of the separation membranes (5) is improved by making the large-diameter bubbles (6) act on the separation membranes. Furthermore, the multilayer provision of bubble entrapment plates (3) enables the bubbles (6) to act on the entirety of the separation membranes (5).
