This study is a development of the exterior diaphragm whose protrusion depth is smaller than conventional ones for applying to the high strength steel box-section column. In this paper, the static behavior of three dimensional cruciform frames with the developed exterior diaphragm is investigated by 45-degree directional loading test. From test results, it is revealed that the elasto-plastic behavior of frames is stable and there is no crack around the exterior diaphragm until the ultimate state. The yield strength obtained from test exceeds the calculated value by the evaluation method of reference [3] .
