Fracture of an underground gas main with a diameter of 530 mm during the production of pneumatic tests along a factory longitudinal welded joint made by contact welding using high-frequency currents, is considered. Fracture research was carried out by analysis of the project and technical documentation of the object, visual and measuring control of the object, ultrasonic measurement of pipe wall thickness, fractographic, metallographic and chemical analyses, measurements of hardness and microhardness of pipe metal. It was established that fracture of the gas pipeline occurred as a result of the opening of one pipe along the entire length of the factory-made longitudinal welded joint during the propagation of cracks in the welded joint and the subsequent propagation of the main crack in both directions to adjacent pipes; the centers of fracture were numerous surface crack-like defects of the welded joint. It is shown that a fracture was caused by a violation of the welding technology of a longitudinal welded joint, a violation of heat treatment, an unacceptable thinning of the pipe wall in the weld zone.