PURPOSE: To improve the quality of a product and to reduce the cost thereof by superposing two sheets of core materials consisting of metallic sheets in an upright state, installing such core materials in a casting mold, preheating the same to a specific temp. or above and subjecting a different molten metal to bottom pouring.

CONSTITUTION: The core materials 8-1, 8-2 consisting of the metallic sheets are superposed and are installed in the upright state to approximately the inside center of the casting mold 7. A pressure vessel 1 is provided below the mold 7 and a sucking pipe 3 is immersed into a molten steel 4. The pipe 3 is connected via a sliding nozzle 6 to the mold 7. The materials 8-1, 8-2 are heated to ≥500°C and the vessel 1 is pressurized by gas, then the molten steel 4 is sucked up and poured into the mold 7 through the pipe 3. The exfoliation of the composite slab is prevented by the preheating of the core materials and oxidation is lessened by gentle pouring. The quality of the product is thus improved and the production cost is reduced by the decreased steps of the stage.