A spiral separation membrane element including a plurality of separation membrane pairs wound around the outer peripheral surface of a water collection pipe, wherein the plurality of separation membranes include at least two sets of separation membrane pairs in which two adjacent separation membranes are stacked such that the water supply sides of the separation membranes form opposing surfaces, and the water supply-side surfaces at the ends of separation membranes close to the water collection pipe and parallel to the lengthwise direction of the water collection pipe are sealed by a sealing material.