Membrane LNG tanks are constructed of a laminate of thin plate invar (Fe-Ni) and a heat-resistant (perlite) box. Invar is 0.5 to 3.0 mm thick -extremely thin compared to the ship structure, and requires highly skilled tungsten inert gas (TIG) welding. Although machine welding centered on relatively simple components has been developed in Europe, complex components continue to require highly skilled welders. We have developed TIG automatic welding for use on the raised edges of tank corners, a type of component requiring high-level welding skills. These welds are highly confined, and the joint shape changes in a complex manner. Key technologies include seam tracking for the weld line, automatic setting of welding conditions, and miniaturization to within 7 kg per unit. The apparatus that we developed is effective for automated, efficient high-level TIG welding, and the first Japanese example of such application in the fabrication of membrane LNG ships is currently being studied.