PURPOSE: To remove easily and safely by-product fine powder of silicon by depositing silicon which is formed by the decomposition of raw gas on a substrate, sucking the gas and the by-product fine powder of silicon remaining in a reaction chamber, and then treating the gas and the powder with a wet system.

CONSTITUTION: A raw gas contg. Si is introduced into a reaction chamber 1, and decomposed. The formed silicon is deposited on a substrate to form a silicon layer. After the completion of said operations, (i) unreacted raw gas and gaseous by-produces in the reaction chamber 1 are introduced into a combustion tower 3 through a discharge device 2 to obtain the oxides which are then collected by a wet dust collecot 4 (fine particulate solids still remaining are removed by a cyclone 6 and a bag filter 7). Then, (ii) the by-product fine powder of silicon remaining in the reaction chamber 1 is sucked by using a suction pipe 10 connected to a flexible pipe 11a (the symbol 11B is a pipeline), sent to the wet dust collector 4, and collected.