TECHNOLOGY FOCUS - POLYMERS - Preferred Membrane: The electrolyte membrane is solid electrolyte membrane. The first electrolyte is a hydrocarbon electrolyte layer, and the second electrolyte layer is a hydrocarbon electrolyte layer having a different chemical formula. The polymer electrolyte membrane is an aromatic hydrocarbon polymer electrolyte having an ion exchange group. The solid polymer electrolyte membrane is polyether sulfone having an ion exchange group (preferably sulfonic acid group). The ratio of the thickness of the first electrolyte layer and the thickness of the second electrolyte layer is 1:10-4:5. The thickness of the first electrolyte layer is 5-40 microns, and the thickness of the second electrolyte layer is 10-50 microns. EXAMPLE - A electrolyte composite membrane was prepared as follows: Sulfonated polyether sulfone with a number average molecular weight of 4x104 and an ion exchange equivalent weight of 8x102 g/equivalent was dissolved in N,N-dimethylacetamide to obtained electrolyte solution (30 wt.%). The electrolyte solution was cast and coated on a glass substrate. A polyolefin porous membrane was placed thereon, and impregnated therewith. Further, an electrolyte solution was cast and coated on it. At this step, by controlling the amount of the electrolyte to be cast, the thicknesses of the electrolyte layers on the opposite sides of the polymer layer were changed. Thereafter, heating and drying were carried out at 80degreesC for 20 minutes, and then at 120degreesC for 20 minutes, to remove the solvent in the solution. As a result, there was manufactured a solid polymer electrolyte composite membrane having a porous layer on the inner sides of a pair of electrolyte layers in which the thickness of one electrolyte layer was larger than the thickness of the other electrolyte layer. The observation of the cross-section of the resulting electrolyte composite membrane indicate that the total thickness of the electrolyte composite membrane was 40 microns.