Abstract: There is provided an improvement of a reverse osmosis separation apparatus in that a membrane module or a permeator has spiral passages for a feed solution and a permeated solution respectively, those passages being formed with membrane sheets about a hollow mandrel to allow the feeding solution to flow spirally on a plane perpendicular to the axis of the mandrel thereby performing with a high recovery ratio and preventing the feed solution from passing through the feed solution passage in an unequally distributed flow state, which decreases rejection of the solution. The improved apparatus has a reinforced module in which the spiral sheets are held together with the mandrel in a rigid state by means of sealing members forming the opposite ends of the module thereby preventing the spiral sheets from being subjected to telescoping or axial deformation which results in damage to the membrane sheets.