In this research, the distortion resulting from stress relaxation was measured by opening a core with strain gauge, which was attached to several places of a full-scale steel beam, and the residual stress was calculated from the measured strains. This research summarized and compared the calculated residual stress distribution. In the material testing using the minute test piece for grasping mechanical characteristics, tensile test and Charpy impact test were conducted. In these each two experiments, two kinds of minute test pieces with different dimensions and joining method that assumed to be manufactured by a core about 20 millimeter in diameter were used. In tensile testing, this study shows that the reduction in board thickness direction, shape of the specimen and initial strain affect the strength and Young’s modules. In Charpy impact testing, this study shows that it is possible to obtain correct Charpy absorbed energy by using welded specimen.