PURPOSE: To improve a hermetic function by disposing a packing in association with the lower edge of a movable plate.

CONSTITUTION: The movable plate 2 is provided with a seal piece 170 extending over the entire length thereof. The seal device 170 is supported by, for example, at least two pressure medium cylinders 171 and has the elongated packing 175 on the side facing a guide piece 172 on which the lower edge of the plate 2 and the upper side 173 of a conveyor belt 9 slide. The packing 175 is formed as a tubular packing and consists of, for example, plastically deformable plastic. When the seal piece 170 is moved to the active position at the time the packing 175 is pressed to the lower edge of a movable plate 2 and the guide piece 172, the pressure is fed immediately in order to enhance a sealing effect. Since the end of the guide piece 172 of the conveyor belt 9 facing the packing 175 is continuously coupled, via, for example, a strip 176, to a stationary plate 1, the gas tight sealing is assured there as well.