A fluoropolymer electrolyte having an equivalent weight (EW) of not less than 250 but not more than 700 and a proton conductivity of not lower than 0.10 S/cm as measured at a temperature of 110°C and a relative humidity of 50% RH and which comprises a repeating unit (α) derived from a SO 3 Z group-containing monomer represented by the general formula (I) : CF 2 =CF(CF 2 ) k -O l -(CF 2 CFY 1 -O) n -(CFY 2 ) m -A 1 (I) wherein Y 1 represents F, Cl or a perfluoroalkyl group; k represents an integer of 0 to 2, 1 represents 0 or 1, and n represents an integer of 0 to 8 and n atoms or groups of Y 1 may be the same or different; Y 2 represents F or Cl; m represents an integer of 0 to 6 provided that when m = 0, 1 = 0 and n = 0; m atoms of Y 2 may be the same or different; and A 1 represents SO 3 Z in which Z represents an alkali metal, an alkaline earth metal, hydrogen atom or NR 1 R 2 R 3 R 4 in which R 1 , R 2 , R 3 and R 4 each independently represents an alkyl group containing 1 to 3 carbon atoms or hydrogen atom, and a repeating unit (β) derived from an ethylenic fluoromonomer copolymerizable with said SO 3 Z group-containing monomer, the content of the repeating unit (α) being 10 to 95 mole percent and the content of the repeating unit (β) being 5 to 90 mole percent, with the sum of the repeating unit (α) content and the repeating unit (β) content being 95 to 100 mole percent, and has an ion cluster structure, wherein the distance between ionic clusters at 25°C and a relative humidity of 50% RH as calculated from the formula given below following small angle X-ray measurement is not shorter than 0.1 nm but not longer than 2.6 nm: d = λ / 2 / sin θm wherein d is said distance between ionic clusters, λ is wavelength of incident X ray used in small angle X-ray scattering measurement and θm is peak-showing Bragg angle. wherein Y 1 represents F, Cl or a perfluoroalkyl group; k represents an integer of 0 to 2, 1 represents 0 or 1, and n represents an integer of 0 to 8 and n atoms or groups of Y 1 may be the same or different; Y 2 represents F or Cl; m represents an integer of 0 to 6 provided that when m = 0, 1 = 0 and n = 0; m atoms of Y 2 may be the same or different; and A 1 represents SO 3 Z in which Z represents an alkali metal, an alkaline earth metal, hydrogen atom or NR 1 R 2 R 3 R 4 in which R 1 , R 2 , R 3 and R 4 each independently represents an alkyl group containing 1 to 3 carbon atoms or hydrogen atom, and a repeating unit (β) derived from an ethylenic fluoromonomer copolymerizable with said SO 3 Z group-containing monomer, the content of the repeating unit (α) being 10 to 95 mole percent and the content of the repeating unit (β) being 5 to 90 mole percent, with the sum of the repeating unit (α) content and the repeating unit (β) content being 95 to 100 mole percent, and has an ion cluster structure, wherein the distance between ionic clusters at 25°C and a relative humidity of 50% RH as calculated from the formula given below following small angle X-ray measurement is not shorter than 0.1 nm but not longer than 2.6 nm: d = λ / 2 / sin θm wherein d is said distance between ionic clusters, λ is wavelength of incident X ray used in small angle X-ray scattering measurement and θm is peak-showing Bragg angle. <EOS>