PURPOSE: A furnace including a window is provided to minimize that a foreign substance including a dust, soot etc. is adhered to a window by arranging a shield plate in a position corresponding to the window. CONSTITUTION: A furnace including a window comprises a housing (130) of a closure structure of being able to open by a door. The furnace includes a window (170), and a foreign substance block unit (200). The window is formed at a constant area of the door or the housing. The foreign substance block unit is moved by uniting with a constant area of the door or the housing. The foreign substance block unit comprises a shield plate. The shield plate is positioned in an internal space formed by combining the door and the housing, and positioned in order to be corresponded to the window.