PURPOSE: To prevent the generation of cracking by putting a billet or ingot into a soaking pit until the solidified shell sticking thereto cracks on shrinkage by a temp. decrease and heating the billet then rolling the same.

CONSTITUTION: A molten steel 1 is stored in a prescribed vessel 2 and is held at a prescribed temp. The carbon steel ingot 3 is immersed for the prescribed period into the molten steel 1 in the vessel 2 by a hoisting means 4 and is then pulled up. The composite billet 5 is conveyed to the soaking pit at the temp. above the threshold temp. for cracking determined by the strength and thermal shrinkage of the hot and particularly stuck solidified shell 1'. The composite billet 5 is charged into the soaking pit where the billet is heated and bloomed to a prescribed size. The billet 5 is cooled down to an ordinary temp. after rolling. The bloomed clad steel bloom is subjected to surface conditioning and grinding to adjust the composite ratio of cladding. The clad steel bloom after the conditioning is heated in a heating furnace and is rolled to a thick plate, by which the intended clad steel is formed.