PURPOSE: To produce a composite steel plate having excellent quality under good yield by arranging the different kind of steel plate in space part between heat radiated cover in a mold and molten steel surface poured from the bottom face of mold and abutting the different kind of steel plate on the poured molten steel surface.

CONSTITUTION: The rectangular mold 2 is put on the base plate 1 and the hot insulating frame 3 is fixed at the side face of mold 2, and blowing holes 4 and supplying pipes 5 for inert gas are arranged at the upper end part. Further, the heat radiated cover 7 penetrating gas exhaust holes 6 at the upper part of mold 2 is put and the different kind of steel plate 9 is arranged as freely descendably in the space part between the cover and the surface of poured molten steel 8 by hanging device 10. Next, the molten steel 8 is poured by the prescribed quantity from the pouring hole 11 to solidify from the bottom face, and the inert gas is flowed from the blowing holes 4. At elapsing the prescribed time after pouring the different kind of steel plate 9 is descended and abutted on the molten steel 8, to deposit and the composite steel plate is produced. In this way, the composite steel plate having good quality is simply produced under good yield.