PURPOSE: To produce inexpensively a hollow steel material by inserting a core casting mold in the central part of an ingot to be drawn in follow up to said ingot, and making the same deliverable up to the position where the solidified body growing around the core casting mold and the solidified layer developing from the outside surfaces joins to each other.

CONSTITUTION: When molten steel is charged into a tundish 2, a dummy bar 6 retreats and casting is started, and before or after said starting, a core casting mold 7 is forced into the molten steel. When the mold is inserted therein until the solidified body formed on the front surface of the bar 6 butts thereto, the plug 9 mounted to the forward end of the mold 7 is captured and fixed according to the growth of the solidified body. The mold 7 is delivered in follow up to the retreat of the bar 6 so that the solidified layer is formed around the same. On the other hand, the solidified layer growing from the outside circumference similarly to ordinary continuous casting joins finally with the solidified layer developed around the mold 7.