Specifically, as shown in FIG. 6, the non-oxidizing gas is injected into the dry type dust collector 3 during the waiting period following the return of the pressure to a positive pressure, using non-oxidizing gas injection piping 64 for injecting a non-oxidizing gas such as a nitrogen gas, an argon gas, etc. to dry type dust collector 3, opening/closing valve 65 and manual or automatic flow rate