PURPOSE: To improve yield of blooming by notching grooves on a mold and a base plate and making the prescribed shape of an ingot.

CONSTITUTION: The groove 7 is notched on the base plate 1 just below a core material in the range surrounded in the inner wall of the mold 2 and the vertical grooves 8 are notched at position facing to both side ends of the core material in the inner wall surface of the mold 2. When the shortest distance between the core material aud groove is large, the danger of development of cracking is eliminated, but as the yield is lowered, it is made ≤50mm. The development of cracking of a slab is eliminated which blooms a cast-in clad ingot cast by this method, and width and length of the cutting-off part is remarkably shortened and the yield of the bloom is improved.