A problem to be solved by the present invention is to provide an
ion exchange membrane that can increase the surface area of a membrane effective for
ion permeation, properly decreases the electric resistance of a flow passage between membranes and the attachment of a contaminating substance thereto, also enhances the
mechanical strength of the membrane itself, and further, is less deformed or damaged due to swelling over a wide range of salt concentrations and even if there is a large difference in salt concentration between two solutions contacted with the membrane. The present invention provides an
ion exchange membrane having a concavo-convex shape, wherein the
ion exchange membrane has a flat portion in the vicinity of ends, and a convex curve and a concave curve resulting from curvatures of the
ion exchange membrane itself form a convex part and a concave parts, respectively, in the concavo-convex shape of the
ion exchange membrane, wherein the convex part extends linearly or curvedly, the concave part between the convex parts is flat, the concave part includes a first concave part adjacent to the convex part in the lateral direction of the convex part, along the longitudinal direction of the convex part, the convex part has an apex and a side face in the longitudinal direction, and the side face is inclined from the apex toward the first concave part.