PURPOSE: To provide a means having better working efficiency in the case of column connection/beam connection even if various kinds of beams having different heights of flanges are connected in a beam connecting section in the case steel pipe columns are connected to each other with a hardware.

CONSTITUTION: An upper side internal fitting section 3A capable of fitting internally and a lower side internal fitting section 3B are formed together as a unit in connection ends 1a and 2a of square steel pipe columns 1 and 2. A reinforced member 6 extending over a beam connection scheduled position along the steel pipe columns is provided to the inside thereof. Projections 3a functioning as a positioning means in the case of fitting internally are provided to the peripheral surface between the upper side internal fitting section 3A and lower side internal fitting section 3B. According to the constitution, in such a state to fit hardwears internally to the connection ends 1a and 2a, the reinforcing member 6 for the hardware can be always placed to a position corresponding to the height of a flange 4a of a beam 4.