To prevent the development of hard spot defect caused by nonmetallic inclusion after casting and to display the degassing effect in the max. limit by fitting a voltex preventing ring at the upper part of a tubular rotary body providing a rotary blade so that the non-metallic inclusion does not entrap into molten metal with the voltex developed with the rotation.

The min. diameter of an L-shaped part at the lower end of the voltex preventing ring 9 is made to 1.0-2.0 times of the outer diameter of the rotary blade 7. Since the lower end part of the voltex preventing ring 9 is formed as the L-shape, the voltex preventing effect is further developed, and then, the size of the projecting part in the inner diameter direction of the L-shaped part is made to 1.0-5 times of the thickness of cylinder part in the voltex preventing ring 9. Further, the dipping length of the voltex preventing ring 9 is made to 0.3-0.5 times of the dipping length of a stirring shaft (rotary body) 2. The material of the voltex preventing ring 9 is desirable to a graphite crucible material from the view point of heat shock resistance, hardly sticking property to metal and erosion resistance.