In thin plate welding, welding deformation is deformation is produced in special form like buckling distortion, which is different from one in thick plate welding, large quantitatively, and has complicated form. Therefore, a particular countermeasure to prevent the welding deformation in manufacturing process is requested. Otherwise it takes more time to straighten the welding deformation than to fabricated a steel structure newly and in case of failing to straighten the welding deformation in beginning of the flame straightening process, even if the flame straitening is completed, the appearances is not good and sometimes eve refabrication is needed. To minimize these problems. In this present paper, the effects of the condition of support pin on out-of plane deformation produced by thin plate Butt welding in investigated through experiment and the countermeasure to prevent the welding deformation in suggested.