PURPOSE: To effectively absorb carbonic acid gas and obtain uniform ground grouting material, by providing a mixing chamber with an outlet port opened in a throttled state, and by mixing the grouting material with the carbonic acid gas in the mixing chamber in a compressed state.

CONSTITUTION: To a mixing chamber 5 having an outlet 6 opened in a throttled state, water glass is fed from the storage tank 1 of grouting material via a feed system A. In the meantime, when carbonic acid gas is fed to a spraying nozzle 10 from a carbonic acid gas high-pressure vessel 7, then the carbonic acid gas is pressure-fed from the nozzle slot 10b of a disc 10a to the mixing chamber 5 and is mixed with the water glass in the mixing chamber 5. The obtained mixture solution is continuously discharged from the outlet 6, and ground is impregnated with the solution as ground improving agent through a receiving tank 13. As a result, the mixing is executed in the mixing chamber 5 retained in a pressed state, and so the carbonic acid gas is effectively absorbed into the water glass at a constant rate, and the uniform grouting material can be obtained.