To prevent energy loss due to take-in and take-out of works by setting up a shutter door provided openingly with a rectangular-shaped slit which takes in and out the work and intermittently lifting and lowering this shutter door so as to take in and out the work to each stage area.

When housing a work in a furnace body 2 or more specifically inserting the work into a shelf frame 3, at first, a shutter door 4 is laid out at the lowermost descent position and its slip 5 is positioned at the lower most stage of the shelf frame 3. An un-processed work loaded on an entrance stage 1a is scooped by operating an automatic arm 9 while the work is housed at the lowermost stage of the shelf frame 3 from the slit 5. Then, the shutter door 4 is intermittently lifted so that the un-processed work may be housed in each stage in the shelf frame 3 from the slit 5 every time the shutter door 4 is lifted. A sheathed heater is energized so that heat may be generated while the heated air is introduced into the shelf frame 3 so as to heat-treat the work by driving a Silocco fan.