PURPOSE: To produce easily and economically a composite steel ingot by forming a slag layer and a metallic bath, by electroslag melting, into the space between an upright core material and an annular water-cooled mold and moving an electrode thereby performing build-up welding on the circumference of the core material.

CONSTITUTION: A circular cylindrical core material 1 made of a defect-free material is set upright, and an annular water-cooled mold 6 is installed around the same so as to enclose the core material. A slag layer 4 and a metallic bath are formed in the space between the material 1 and the mold 6 by using electroslag melting. While an electroslag melting electrode 5 is moved around the material 1, the welding position is moved successively upward to perform build up welding around the material 1, whereby a composite steel ingot is produced. The molten metal in the initial period of melting is supported by a stub 8 and the solidification in the upper part of the slag is prevented by a nonmetallic refractory material 7.