PURPOSE: A method is provided which coats MgO contained slag on the surface of refractory materials in a vacuum vessel of refining facilities by spraying an inert gas having certain temperature and pressure onto the slag after ascending molten steel and slag to a certain elevation. CONSTITUTION: In a molten steel slag coating method for protecting inner refractories refluxing the molten steel and refractories of submerged pipes so as to refine molten steel contained in a ladle, the method for coating slag on surface of refractory materials in vacuum vessel of refining facilities comprises the processes of floating slag(9a) through submerged pipes(2,3) submerged into the molten steel(9) of the ladle(8) by forming vacuum inside the vacuum vessel(1); spraying a high pressure inert gas a temperature of which is increased to correspond to an internal temperature of the vacuum vessel(1) from the inside of the vacuum vessel(1) to the slag(9a); and simultaneously scattering the slag(9a) by a spraying pressure of the sprayed high pressure inert gas and increasing an inner pressure of the vacuum vessel(1) so that the slag is coated on the inner refractories of the vacuum vessel(1) and the refractories of the submerged pipes.