PURPOSE: To improve remakably economy and productivity by providing a cylindrical refractory material penetrating through an immersion nozzle 4 on the bottom of a tundish and forming a core casting mold penetrating therethrough so as to have the length down to the depth at which a hollow solidified body grows to a substantial thickness.

CONSTITUTION: A cylindrical refractory material 5 penetrating through an immersion nozzle 4 mounted on the bottom of a tundish 2 is provided in a continuous casting installation of a hollow casting ingot. A core casting mold 6 penetrating through said material 5 is provided. Said mold 6 is formed to have the length down to the depth at which the hollow solidified body formed by the molten metal charged into the tundish 2 and flowed down in the annular spacing between the nozzle 4 and the material 5 grows to a substantial thickness.