Fine-sized magnesium metaphosphate, calcium metaphosphate, or zinc metaphosphate glass powders were reacted with distilled water and hydration immediately occurred,resulting in the formation of hydrogels with high viscosity. ³¹P MAS-NMR spectra showed that the hydrogels contain orthophosphates and long-chain phosphates coordinated to H⁺ or M²⁺(M=Mg, Ca, Zn) ion; no significant differences in their structures among the hydrogels were shown. The hydrogels showed high proton conductivities of ~5 mS/cm at 30 °C. The conductivities were suggested to originate from proton conduction due to coexistence with large amounts of acidic P-OH groups and water molecules. Electric double-layer capacitors (EDCs) were prepared by using electrolytes consisting of the hydrogels. They showed no oxidation-reduction processes in the range of 0~1 V. The EDCs showed specific capacity of ~2 F/g. The possibility as an EDC electrolyte may be related to proton conductivities of the hydrogels.