PURPOSE: To obtain a purifying agent capable of removing impurities from the raw material gas of semiconductor, in high efficiency, by depositing a hydrogenated amorphous substance selected from Si, Ge, P and As to a carrier having large surface area.

CONSTITUTION: A carrier 2 having large surface area is packed in the reaction tube 1, and heated at 300W450°C with the heater 3. The raw material gas for semiconductor (e.g. monosilane gas) is introduced through the three-way valve 11 into the reaction tube 1 and thermally decomposed into amorphous Si and H2. The hydrogenated amorphous Si is deposited to the surface of the carrier 2 to provide the objective purifying agent, and the H2 is discharged from the purging line 16. The three-way valve 10 is turned to connect the reaction tube 1 with the moisture-adsorption column 4 filled with an adsorbent 5, the temperature of the adsorption column 1 is lowered below the decomposition temperature of the monosilane, and the monosilane is passed through the three-way valve 11, the reaction tube 1, the three-way valve 10 and the moisture-adsorption column 4 in the order and the impurities are removed to obtain purified silane.