TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred film : The cation exchange film is biaxially stretched by drying the film containing a large amount of water with the perimeter fixed . The film contains 1 - 50 wt% of fibril, woven cloth or non-woven cloth. Preferred polymer : The perfluorocarbon polymer is a copolymer containing CF2=CF2 units and CF2=CF-(OCF2CFX)m-Op-(CF2)n-SO3H units.X = F or CF3;m = integer between 0 and 3;n = integer between 0 and 12;p = 0 or 1, and when n = 0, p = 0Preferred fuel cell : The fuel cell comprises the cation exchange film composed of the perfluorocarbon polymer containing sulfonic acid group as an electrolyte, and gas-diffusible electrodes on both sides of the electrolyte. Preferred process : The fuel cell is produced by setting the 5 - 100 % stretched polyfluorocarbon film as the electrolyte, and placing the gas diffusible electrodes on the both sides of the electrolyte. Another fuel cell is furnished with a laminate prepared by laminating stretchable films and biaxially stretching them. EXAMPLE - A copolymer comprising a pefluorocarbon polymer having a sulfonic acid group, was obtained by polymerizing 1,1,2-trichloro-1,2,2,-trifluoro ethane, alpha, alpha'-azobisisobutylonitrile and CF2=CFOCF2CF(CF3)O(CF2)2SO2F, and then extruded into a film ( 50 microns thick ). The film was hydrolyzed with an aqueous solution of dimethylsulfoxide and KOH and soaked in a 1N-HCl solution for 4 hr. The film was dried at 90 degrees C under a humidity of 10 % or lower with the perimeter fixed so as to obtain a cation exchange film. A fuel cell was prepared by preparing a coating solution containing the above copolymer and Pt-carrying carbon ( wt ratio = 1 : 3 ), coating the solution on a carbon cloth and drying so as to obtain a 10 microns thick gas diffusible film carrying 0.5 mg/cm2 of Pt. A laminate comprising the copolymer film sandwiched with two gas diffusible films was pressed using a plane presser so as to obtain a film-electrode jointed body. A fuel cell was prepared by setting a Ti-electric collector, a gas supplying chamber made of PTFE and a heater outside the above film-electrode jointed body and had an effective film area of 9 cm2. The fuel cell had excellent durability and fewer wrinkles.