An object of the present invention is to provide an atmosphere heat treatment apparatus that is operable without relying on experiences and/or intuitions of an operator, and a method of operating the same. The atmosphere heat treatment apparatus includes an entrance side exhaust path (26) for exhausting an inside atmosphere from an entrance side of a carburizing chamber (10) to which a transforming gas is supplied, an exit side exhaust path (35) for exhausting the inside atmosphere from an exit side of the heat treatment chamber, and an opening degree adjusting lid (28) attached for adjusting an opening degree of the entrance side exhaust path (26), in which a part or whole of the exit side exhaust path (35) is constituted of a pipe (46) having a predetermined inside diameter. It is checked in advance a relationship of a supplying amount A of the transforming gas with a ratio B:C of an exhaust amount B from the exit side exhaust path (35) and an exhaust amount C from the entrance side exhaust path (26) when a pressure in the carburizing chamber (10) is adjusted to a predetermined furnace pressure by adjusting an opening degree of the entrance side exhaust path (26) with the opening degree adjusting lid (28), and when treating a work (W), the supplying amount A of the transforming gas and a furnace pressure are adjusted so as to control the ratio B:C of the exhaust amount B from the exit side exhaust path (35) and the exhaust amount C from the entrance side exhaust path (26) to be within a predetermined range.