PURPOSE: To provide such a structural central-pillar that steel materials with a shape having the same sectional secondary radius as that of each of X and Y axes are used, sectional area thereof can be reduced in comparison with wide flange shapes and that it can be manufactured at a low cost.

CONSTITUTION: Steel materials (box-shaped or circular steel pipes) having an enclosed section are used for a structural central-pillar to reduce its sectional area and to increase its strength. In order to transfer load (mentioned above) to the structural central-pillar, a shear connector 8 for a reinforcement 4, a plate 5, shape steels 6 and 7, a stud dowel, etc., is provided thereto. The structural central-pillar 1 is such that the steel materials having an enclosed section or the same sectional secondary radius as that of each of X and Y axes are used, it has smaller sectional area in comparison with wide flange shapes and that it is manufactured at a low cost. The transference of load (mentioned above) to the structural central-pillar 1 is made by the shear connector provided to the structural central-pillar 1 instead of adhesion between concrete and steel (structural central-pillar 1 and holes 3).