PURPOSE: To reduce the vol. in a detecting range and to facilitate the decision of cleanliness of a steel by acceleratedly rotating a cylindrical mold until molten steel therein forms as the cylindrical state and cooling and solidifying the molten steel while separating non-metallic inclusion from the molten steel.

CONSTITUTION: The molten steel is poured into the cylindrical mold 1 and an air discharging hole 9 is opened and an inert gas cylinder 12 is connected with a gas supplying hole 10 to displace the air in the cylindrical mold 1 into Ar gas. The cylindrical mold 1 is rotated with a motor 7 and accelerated until the molten steel forms as the cylindrical state while giving the centrifugal force. The molten steel is heat-radiated and cooled to form solidified shell and further, the solidification is progressed toward the inner part. The non-metallic inclusions mingled in the molten steel and having small density are separated from the molten steel and solidified while shifting and accumulating as layer- state in the inner part to make the cylindrical sample. Since the inclusions in this sample are accumulated in the inner part and the vol. of the detecting range can be reduced, the cleanliness of the steel can easily be decided.