PURPOSE: To manufacture a tube by the centrifugal casting free from defects on the inner surface of the tube, and excellent in the yield by specifying the content of Al2O3, CaO, CaF3, MgO and SiO2.

CONSTITUTION: The best way to prevent generation of TIN, Al2O3 and TiO2 causing generation of lump-shaped scum is to completely shut off the molten metal surface from the air. The molten metal can not be completely shut off from the air by the heat generating flux using the thermit reaction which has been the main stream, The cause is that the sufficient viscosity is not obtained because the melting point of the flux is high, and it is effective to adjust the blending of the flux ingredients, and to generate the composite oxides to drop the melting point in order to provide the flux of low viscosity. The flux of low viscosity for the centrifugal casting has the composition consisting of, by weight, 20-30% Al2O3, 10-18% Cab, 10-20% CaF2 20-30% MgO, and the balance SiO2 with inevitable impurities.