PURPOSE: To obtain the observation window capable of observing the contents of a vessel always in a sound state by providing a protective plate mechanism having a protective plate fixed to the peripheral part of a rotating disk in the middle of a cylinder which is fixed to the opening of the vessel, wherein molten materials and chemicals are splashed, and has a transparent plate fixed to its outermost part.

CONSTITUTION: An observation window 7 is fixed to the opening 9 of a vacuum vessel 14 through a gate valve 17. A protective glass mechanism 15 is arranged on the window 7, and a rotating mechanism 13 is provided. The leading end of the rotating mechanism 13 is fixed to the protective glass mechanism 15 through a disk 18 provided with a protecting glass 16 and a driving shaft 19. A window glass 8 is furnished above the protective glass mechanism 15 in conformity with the opening of the vessel 14. When the metal vapor 5 is deposited on the protective glass 16 which is thereby dimmed, the glass 16 is rotated by the mechanism 13 to supply its fresh surface, and observation is performed always in a sound state.