To clarify phenomena which cause strength decrease in the molten pool during laser welding processes, we have carried out X-ray imaging of inside materials using intense synchrotron radiation during laser welding. An air-cooled continuous wave fiber laser was used for welding. When the laser beam was irradiated on sample materials, molten pool shape were successfully observed using an X-ray absorption contrast technique. Moreover, bubbles in molten pool and cracks in weld metal were clearly observed. Butt welding for heat exchanger tubes was demonstrated by a specially designed probing system. The probing system consisted on a composite-type optical fiber, a laser processing head, air-cooled fiber laser and so on. Here, the surface of the weld bead was observed by fiber scope images. In such a repair welding, the observation of the inside material by X-ray imaging is very effective because it can inform us how the weld depth could grow and why the weld defect would be born. DOI: 10.2961/jlmn. 2012.03.0002