PURPOSE: To detect defects, etc. in the weld zones of pipes synchronously with pipe producing stages without making the stages longer by detecting the presence or absence of gaseous leakage in the respective parts of the pipes by scanning the pipe longitudinally through a casing.

CONSTITUTION: A pipe 3 connected of a seam 6 by a welding machine 5 is inserted into dies 2a, 2b and is passed through a casing 1. A fine pipe 4 contained in the pipe 3 is disposed in such a way that its leading end part 7 is located within the casing 1 and its opening face 8 faces a weld zone 9. Thence, a vacuum pump is connected to a channel 10 provided in the casing 1 to evacuate the inside of the casing 1; at the same time, a gas is supplied to the pipe 4 and is flowed out through its leading end part 7. In such a state, the pipe 3 is taken off in the rightward direction, and if there are no defects or flaws in the weld zone 9, the gas in the pipe 3 is not sucked into the casing 1, but if any defect or the like exists, it turns out that the gas is sucked into the casing 1. The sucked gas is sucked into the channel 10, and is detected with the gas detector connected thereto.