Abstract: A pyrogenic oxidation device (10) is comprised of a process gas supply line (38) connecting an external combustion device (39) and a supply pipe (37) connected to a processing chamber (13) and, a dilute gas supply line (45) connected to the process gas supply line (38) for supplying nitrogen gas (62), a purge gas supply line (47) for supplying nitrogen gas (62) and connecting to the exhaust pipe (37) side of the section connecting with the dilute gas supply line (45) in the process gas supply line (38), and a vent line (49) for exhausting gas and connecting to the dilute gas supply line (45) side of the section connecting with the purge gas supply line (47) in the process gas supply line (38), and stop valves (46), (48), (50) in each line opened and closed by a controller (60). A deterioration in film thickness uniformity due to residual matter can be prevented, since residual matter in the process gas supply line is prevented from flowing into the processing chamber during the purge step.