A cation-exchange membrane for electrolysis which comprises a fluoropolymer having ion-exchange groups and a porous base. It is characterized by having, on the anode-side surface of the membrane, protrusions comprising a polymer having ion-exchange groups. It is further characterized in that: when the average value of the heights of the tops of the protrusions from the anode-side surface of the membrane is expressed as h (μm), then 20≦h≦150; when the density of the protrusions distributed is expressed as P (protrusions per cm2), then 50≦P≦1,200; when the average proportion of the areas of those bottom parts of the protrusions which are on the same level as the anode-side surface of the membrane to the area of the anode-side surface of the membrane is expressed as S (cm2/cm2), then 0.001≦S≦0.6; and when the average proportion of the areas of the top parts of the protrusions to the area of the anode-side surface of the membrane is expressed as T (cm2/cm2), then T≦0.05.