To efficiently manufacture a column core with high quality by forming bulge sections in the axial direction and horizontal direction for connecting the flanges of steel frame beams at the upper and lower end positions of a tubular body having the same height as the height between the upper and lower flanges of the steel frame beams.

Bulge sections 10, 11 are integrally formed at the upper and lower end positions of a tubular body having the diameter nearly equal to that of steel pipe columns 4, 5 and the height equal to the height (h) between the upper and lower flanges of steel frame beams 2, 3 to form this column core 1. The bulge sections 10, 11 have the thickness (t) in the axial direction and the swelled quantity (n) in the horizontal direction required to connect the upper and lower flanges 20, 21, 30, 31 of the steel frame beams 2, 3, and bevelings 10a, 11a are formed respectively. When multiple steel frame beams different in beam height are to be connected, multiple bulge sections are formed at the corresponding intermediate sections. Multiple steel frame beams different in beam height can be simply connected by welding in a downward attitude. The column core 1 can be efficiently machined with high quality at a factory, and the construction period can be shortened at a low cost.