Abstract: Disclosed are immobilized electrolyte membranes having a high electric conductivity and a high mechanical strength, where an ionic conductor such as a polymer electrolyte or an ion-exchange resin is immobilized in pores of a porous solid polymer membrane, particularly of polyolefin having a weight average molecular weight of not less than 5.times.10.sup.5, the porous solid polymer membrane having a thickness of 0.1 .mu.m to 50 .mu.m, a porosity of 40% to 90%, a tensile strength of not less than 200 kg/cm.sup.2, and an average permeable pore diameter of 0.001 .mu.m to 0.1 .mu.m.