The invention, for the furnace, such that the oxygen concentration is directly or indirectly detected from any one portion or both portions of the central processing section and the inlet portion and the outlet portion of the furnace according to reflux soldering oven. The signal generated by these sensors are processed in a PID (proportional, integral, differential) controller, by generating a control signal for controlling the flow rate of the inert gas flowing into the central processing portion, which at least air from entering the central processing section to inhibit. PID controller is programmed to be set equal to the time average of the integral error term is the oxygen concentration in the small central treatment area than the oxygen concentration set values ​​required for the central processing section. Proportional error term and a differential error term is set equal to the time average of the concentration of one part or both parts of the central processing section and the inlet section and outlet section of the oxygen concentration is less than the set value, no.