PURPOSE: To reduce an installation space and to facilitate check and maintenance by moving a door body by rotating a hanging member attached at one end to the body by a rotary driver, operating a protective cover as its counterweight, and sealing with a sealing ring the peripheral wall of an opening.

CONSTITUTION: Drivers are operated to rotate a lever 15 for pressing a door body 13 to a sealing ring 7 of the peripheral wall of a conveying opening 6, at a predetermined angle to be opened. Then a motor with a reduction gear brake is operated to rotate a rotary drive shaft 8 to raise the body 13, and to simultaneously lower a protective cover 14. When the cover 14 is moved to a position opposed to the ring 7, the motor is stopped. The cover 14 is pressed to the ring 7 by the rotation of the lever 15 by the drivers. Influence of radiation heat when a material 4 to be treated is passed through the opening 6 is effectively prevented by the cover 14. The cover 14 is also operated as a counterweight of the body 13. As a result, the load of the motor can be reduced.