Provided is a spiral membrane element that has a restricted outer diameter and is capable of being decreased in operation energy therefor. The element is a spiral membrane element including plural membrane leaves in each of which a permeation-side flow-channel member is interposed between opposed separation membranes a supply-side flow-channel member interposed between any two of the membrane leaves a perforated central pipe on which the membrane leaves and the supply-side flow-channel member are wound and a sealing part that prevents a supply-side flow-channel member from being mixed with a permeation-side flow-channel member. This element has an efficiency index E of 0.005 to 0.10, and the thickness of the supply-side flow-channel member is from 10 to 110 mil.