A separation membrane support substrate characterized by being composed of a laminated nonwoven fabric comprising a front layer as the resin coating layer, a middle layer and a back layer which are integrally formed by heat bonding, and by satisfying the following conditions (1) to (5): (1) The front layer has at least one layer comprising thermoplastic resin filaments with a single filament diameter of 7-30 .mu.m; (2) the middle layer has at least one layer comprising melt blown fibers with a single fiber diameter of no greater than 5 .mu.m, and a fiber basis weight of at least 1 g/m.sup.2 and comprising no more than 30 wt % of the total fiber basis weight; (3) the back layer has at least one layer comprising thermoplastic resin filaments with a single filament diameter of 7-20 .mu.m, and has a fiber basis weight of 3-40 g/m.sup.2; (4) the laminated nonwoven fabric has an apparent density of 0.67-0.91 g/cm.sup.3; and (5) the laminated nonwoven fabric has a thickness of 45-110 .mu.m.