To provide a clean oven capable of a cleanliness at a high level by suppressing dusting from a packing without adopting a special structure in the oven even at a high temperature of 500°C and in the rising and lowering of the temperature and enabling the easy replacement of the packing without dusting in the replacement of the packing.

A heat resistant packing 8 formed of a stainless steel fiber is used as the packing interposed between an blowing air inlet frame 7 forming a blowing air inlet in a treatment chamber 2 and a heat resistant filter. Thus, the production of particles at the high temperature, in the rising and lowering of the temperature, and in the replacement of the packing can be suppressed.