A thermal processing apparatus for thermally processing a substrate in a processing chamber includes a heating plate for mounting and thermally processing the substrate thereon, and a lid body covering the heating plate from above to constitute a portion of the processing chamber. The lid body has a ceiling plate and a peripheral side portion vertically provided at a peripheral end portion of the ceiling plate. The ceiling plate is provided with a supply port for supplying a gas into the processing chamber, and a side portion of the ceiling plate is provided with a plurality of exhaust ports for exhausting the gas in the processing chamber. An exhaust pipe communicating with the exhaust ports and having an outlet at a point at equal distances from the exhaust ports is provided to be attachable and detachable to/from the lid body. According to the present invention, the maintenance work against clogging due to the impurities such as a sublimate and the like generated by the thermal processing can be easily performed.