is noted that at least one ignition source is used so as to maintain flames 31 at a suitable position below a nozzle of the burner 23 a water spray tube 30 is provided in such a manner that it is positioned below the flames 31 generated below the burner 23 and as best seen from fig3 is of a u shaped plane and positioned horizontal the opposite tube portions 30a and 30a of the water spray tube 30 are provided on their insides with a number of water injection nozzles 30b adapted to inject water horizontally or downwardly the combustion system of the present invention as constructed above operates as follows first of all a fuel gas such as propane is fed to the pilot burner 29 for ignition while an amount of air is discharged from the air inlet port 21 to an exhaust pipe 50 by way of the combustion gas discharge port 22 to form a downward flow of air within the cylindrical body 20 it is then desired that the flow rate of air is at least 0 5 m sec at a lower flow rate of air difficulty is encountered in discharging fine dust comprising silicon dioxide etc from the system since it tends to be blown up within the cylindrical body 20 owing to its lightness subsequently the combustible exhaust gases containing silane etc from the semiconductor production step the inert gas such as.