To scrape slug from a slug drapper to the outside of a container with high-efficiency.

An origin x0 is determined in the vicinity of an inner wall 5a below a discharge port 4. Based on the position of the origin x0, a scrape-out starting port x1 and a scrape-out completion point x2 positioned outside a pot 5 and in a given position in the vicinity of a mouth 4a are determined. Slug is discharged in such a way that a scrape-out plate 2 is moved backward from the scrape-out starting point x1 to the origin x0 and after it passes the origin x0, it is moved back and stopped once, and the scrape-out plate is raised as it is moved backward to the scrape-out completion point x2 along an inner wall through intermittent operation to alternately repeat a rise to height Δz and a stop during a time Δt. The height Δz and the time Δt are set in such a manner to satisfy formula of Δz height from a level of a molten metal to the mouth and ΔtΔx/v, wherein Δx is a distance to horizontally couple together a perpendicular from a scrape-out origin x0 and a perpendicular from the scrape-out completion point X2, and V is the backward movement speed of the scrape-out plate.