PURPOSE: To improve the contact between a casting and water-cooled copper plates and to cast a steel ingot having no internal defects in the stage of casting the steel ingot with an upward expanded casting mold consisting of long-side plates and short-side plates by attaching the water-cooled copper plates to the inside of the long-side plates and making the same movable toward the inside.

CONSTITUTION: Water-cooled copper plates 10A are attached to the inside of the long- side plates 8 of a casting mold formed of the long-side plates 8 and short-side plates 12 so as to expand upward at an angle θ on the double molding boards 2B on a molding board 2A. A clearance 11 is provided between the bottom end thereof and the boards 2B and is kept closed with asbestos, etc. A molten steel 32 is cast by bottom pouring through a pouring pipe 4 and a runner 6 into the casting mold. The molten steel begins to solidify from the surface in contact with the boards 2B and the plates 10A and forms a solidified shell 34. The steel ingot shrinks on progressing of the solidification and a clearance is made between the shell 34 and the plates 10A and therefore the plates 12 are moved upward to form a spacing from the plates 8. The plates 8 are pressed by hydraulic cylinders 30 to bring the plates 10A into contact with the solidified shell at all times. The decrease in the cooling speed of the steel ingot is obviated and the steel ingot having no internal defects such as reverse V segregation, porosity, etc. is produced.