TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Film: The porous base material contains at least one aromatic polyamide and has a mean pore diameter of 0.01-1 microns, a porosity of 20-80 % and a thickness of 5-300 microns. The base material has a thermal resistant temperature of 200degreesC or higher and upon heat treatment at 105degreesC for 8 hours it shrinks less than 1 %. The porous base material has a polymer phase/space net structure which forms continuous holes and a porous structure on both surfaces. A third polymer capable of conducting protons is further filled in the pores of the base material. The electrolyte film changes its area between the dry state and the wet state at 25 degreesC, by 1 % or less. The electrolyte film has a proton conductivity, at a temperature of 25 degreesC and a humidity of 100%, of 0.001-10.0 S/cm.Preferred Cell: The fuel cell is a solid polymer type fuel cell which is a direct-type methanol solid polymer fuel cell.Preferred Process: A process for producing the electrolyte film comprises filling an electrolyte substance composed of monomers, which form a polymer capable of conducting protons, in the pores of a polyimide porous film and then heating it so as to polymerize the monomers in the pores. The overall filling rate is controlled by repeating at least once after the first heating the heating to achieve polymerization. A process for filling the electrolyte in the pores of the base material comprises filling the electrolyte substance by any one step, any two-step combination, any three-step combination or by all four steps of the following four (X-1) -(X-4) steps after polymerization of the monomers in the pores by heating and/or carrying out (Y-1) and/or (Y-2) steps after filling the electrolyte in the pores (X-1).(X-2) includes making the porous film hydrophilic and then soaking the resulting film in a monomer or its solution.(X-3) includes adding a surfactant to a monomer or its solution and soaking the porous film in the resulting solution.(X-4) includes decreasing the pressure over the porous film being soaked in a monomer or its solution.(Y-1) includes irradiating the porous film being soaked in a monomer or its solution with a supersonic wave.(Y-2) includes bringing the porous material that is to absorb the electrolyte into contact with both surfaces of the porous film.(Y-3) includes removing any excess electrolyte fixed on the porous film by smoothing.The polyimide contains 3,3',4,4'-biphenyltetracarboxylic acid dianhydrate and oxydianiline. The porous film does not swell in methanol or water. A radical polymerization initiator is added together with the surfactant to the monomer or the monomer solution. WIDER DISCLOSURE - A fuel cell has the electrolyte film. An electrolyte film/electrode jointed body comprises the electrolyte film. A fuel cell comprises this electrolyte film/electrode jointed body.    EXAMPLE - A polyimide film was soaked in a solution containing 70 wt% of a mixture comprising acrylic acid (79 mol%), vinylsulphonate-Na (20 mol%) and divinylbenzene (1 mol%) while irradiating with visible light for 6 minutes and drying at 50 degreesC for 18 hours. Excess polymer was removed from the surfaces of the film. The film was subjected to ion-exchange using an excess amount of a 1N-HCl solution, washed with distilled water and dried at 50 degreesC so as to obtain an electrolyte film having excellent proton conductivity and inhibited methane permeation.