The present invention relates to an exhaust gas treatment unit which possesses a stable treating capacity, with a reduced running cost. The exhaust gas treatment unit removes noxious gases such as monosilane contained in an exhaust gas by thermal decomposition. The exhaust gas treatment unit includes a heating drum (5) into which the exhaust gas and oxygen or air are introduced and a heating source (6) disposed on the outer periphery of the heating drum (5) so as to heat the inside of said heating drum (5). A temperature sensor (14) detects the temperature of the inside of said heating drum (5) in the vicinity of its outlet and a controller (16) controls the heating source (6) in accordance with a detected temperature value from the temperature sensor (14) so that the temperature of the inside of said heating drum (5) in the vicinity of the outlet is maintained within a predetermined temperature range. An air-cooled type cooling device (2) is connected to the outlet of the heating drum (5) to cool the exhaust gas. The running cost of the invention is lower than the running cost for other known devices in a large extent because metal oxide and chemical agents are not used in contrast to a chemical reaction type exhaust gas treatment unit.