To increase safety and reduce costs in maintaining buried pipelines, we have been studying various types of robots capable of inspecting and repairing such pipelines from inside, and we have recently succeeded in developing automatic welding robots to reinforce welds from the inside of a steel pipe with a diameter of 600 mm. It is now within our scope to put it into practical use. This paper presents a general description of three such robots models which we have developed. These robots, designed to travel over more than 100 m in a complex of piping with bends, can do welding to reinforce existing welds. This paper chiefly discusses techniques used in our development to solve the most difficult problems, namely the determination of the most appropriate travelling and welding methods. Through our activities, we have obtained useful findings to further the study of robotic systems for pipeline inspection and repair.