This study focuses on the development of a new method of H-shaped beam-to-square column connection with outer diaphragm with field welding. The specific type of beam-to column connection with external stiffener using field welding is proposed. The structural behavior of this connection is examined experimentally. The two loading type tests are conducted under the experimental parameters given as details. First are described symmetrical loading test that supported the both ends or a beam simply and applied load from the column top end for investigating a fundamental characteristic of this connection. Further are given the anti-symmetrical loading test that carries out simple support of the column top end and the column base, and applied load from the both ends of a beam for investigating structural performance of this connection. From the results, it makes clear that the external stiffener type connection proposed in this paper is reliable connection method.