Structure and method for joining column and beam

In this structure for joining a column and a beam, an H-shaped steel beam is joined to a column by an outer diaphragm. The outer diaphragm comprises a divided diaphragm that is divided into a plurality of pieces. The divided diaphragm comprises a column plate that is brought into contact with the column. A piece of the divided diaphragm among the plurality of pieces of said divided diaphragm that is arranged at a position that corresponds to the H-shaped steel beam is spliced to a flange of the H-shaped beam and comprises a beam plate that has the column plate provided to an end section thereof. The individual pieces of the divided diaphragm are fastened and fixed to one another via a joining member so that contact pressure from the column plate acts on the column surface of the column.