PURPOSE: To improve the sealing characteristic of a vacuum treating chamber as much as possible by providing a seal box having a passage port for a material to be treated on the inlet and outlet sides of a vacuum treating apparatus and opening and closing the passage port by a sealing plate which is divided in accordance with the width of the material to be treated.

CONSTITUTION: A sealing device 20 is respectively provided to the inlet side (i) and to the outlet side (o) of a material 4 to be treated in a vacuum treating apparatus 66. The sealing device 20 is composed of a seal box 22 having a passage port 21 for the material 4 such as band steel, a sealing plate 23 which is divided in accordance with the width of the material 4 and used for opening and closing the passage port 21, a roll 68 for preventing the deflection of the material 4 and an evacuation means 25 for evacuating the inside of the seal box 22. The sealing plate 23 is formed by the central sealing plate 23a, side sealing plates 23b and 23c on both sides of the plate 23 and the outer sealing plates 23d and 23e on the outermost side. The sealing plates 23aW23e are appropriately moved up and down by a rod 32 through a rotary driving shaft 31 driven by a stepping motor 30 in accordance with the width of the material 4 and the passage port 21 for the material 4 is opened. The pressure of the seal box 22 is reduced to a prescribed pressure by the evacuation means 25.