PURPOSE: To manufacture a steel bar at high efficiency, by continuously casting an ingot with an irregularly shaped cross section obtained by horizontally joining bottom planes of plural trapezoidal cross sections, by a rotary caster, and by splitting this ingot after cooling and straightening, into individual billets.

CONSTITUTION: A molten steel 18 is poured from a tundish through a submerged nozzle 12 into a space having an irregularly shaped cross section obtained by horizontally jointing plural adjoining trapezoidal bottom planes. Only outer ends of cross sections on both ends are closed, and the molten steel 18 is forced by a belt 3 and a casting wheel 2 to flow along the bottom plane of the cross section. An ingot 19, being cooled and upheld by a cooling guide, is straightened by a leveling roller, and is extracted by a pinch roller 6 and a supporting roll 14. During this period of extraction, a water spraying nozzle applies the secondary cooling; and the ingot 19 with an irregularly shaped cross section obtained by horizontaly jointing plural adjoining trapezoidal bottom planes, is continuously extracted; the splitting shear 7 splits the jointing part of each trapezoidal shape; hereby, plural straight billets 19 with a trapezoidal cross section are obtained. These billets 19 are then directly subjected to a hot rolling pass after passing a warming furnace.