A wafer loading and unloading chamber is provided at the bottom of a reaction tube of a heat treatment section, a robot chamber and cassette chamber are coupled via gate valves to the wafer loading and unloading chamber, the robot chamber comprises a first load lock chamber while the wafer loading and unloading chamber comprises a second load lock chamber. These first and second load lock chambers are mutually connected by a pressure balancing gas conduit via pressure balancing valves. During heat treatment of the target objects, the wafer loading and unloading chamber and robot chamber are first set to inert gas atmospheres, then the pressure balancing gas conduit valves are opened to balance the pressures of the robot and cassette chambers. Gate valves between these chambers are then opened, and the cassette is conveyed via the robot chamber to a wafer boat in the wafer loading and unloading chamber. By this process gas flow between the two chambers when the gate valves are opened is suppressed to allow minimizing the production of particles.