PURPOSE: To automatically set level difference between heights of a ladle and ladle spout and molten metal surface.

CONSTITUTION: This instrument is constituted of a detecting instrument for height of the ladle spout hanging down a detecting member 4 for level of the ladle spout from upper part of the ladle and forming this member 4 as freely lifting in the prescribed range according to lifting of the ladle and transmitting a signal in the prescribed level and a detaching instrument 23 for molten metal surface arranging a detector 23 for molten metal surface level at lower part of the above detecting member 4 for height of the ladle spout level and transmitting a signal at the time of bringing this detecting instrument 25 into contact with the molten metal surface. The ladle is lifted up until the detecting instrument for height of the ladle spout works, and after that, by tilting the ladle until the detecting instrument for molten metal surface works, the ladle spout can be automatically set to position matching to level of scraping-out system slag-off machine.