In case of non-diaphragm type beam-to-column connection, out-of-plane deformation of column flange occurs and has some effect on overall behavior of steel frame. This paper investigates mechanical behavior of non-diaphragm type beam-to-column connection using thick square hollow section by means of full-scale loading test and finite element method analysis. As a result, it is clarified that distribution of shear force at panel zone of non-diaphragm type is differnt from that of through-diaphragm type, and strength of the non-diaphragm panel decreases and elastic stiffness increases.