Abstract: A heat treating apparatus for mounting objects to be treated on an object-to-be-treated boat (wafer boat) provided on a heat insulating cylinder, and loading the object-to-be-treated boat into a processing vessel for a heat treatment, a temperature detecting sensor is provided in a film depositing area of a relatively large heat capacity, which is a lower part of a gap defined between an inner tube of the processing vessel and an outer tube thereof so as to detect temperatures of those of the objects to be treated located there and control temperatures of the heating unit. Thus heat response is improved, and temperatures of the objects to be treated can be accurately detected.