A novel technique was developed to inject proton carriers into phosphate glass. Sodium ions in the sodium tungsten phosphate glass were electrochemically substituted with protons. A glass plate with a deposited Pd anode was placed on the molten tin cathode and heated at a temperature below glass transition temperature under DC-bias in hydrogen containing atmosphere. Protons were injected from the palladium anode and substituted the sodium ions in the glass. The sodium ions migrated to the cathode and discharged to the molten tin cathode by capturing an electron from the external circuit. The atomic sodium dissolved into the cathode and reacted with CO2 impurities in the atmosphere resulting deposition of sodium carbonate on the tin cathode surface. After almost all of the sodium ions were discharged from the glass, the proton concentration in the glass reached 4.3 × 1021 cm-3. The proton conductivity was found to be 1 × 10-3 Scm-1 at 350°C. The injected protons were stable at temperatures below 400°C. The glass obtained by this technique behaves as a mixed conductor of protons and electrons with a proton transport number of 0.30. A possible means to suppress the electronic conductivity was discussed.