PURPOSE: To prevent the thickness deviation of a pipe by providing plural non- contact type temp. sensors and nozzle at opposing position in peripheral direction of a welded pipe and by regulating the cooling water volume of an optional nozzle.

CONSTITUTION: A non-contact type temp. sensor 8 and thermometer 9 are arranged at equal intervals in peripheral direction to an annular member 7 provided with surrounding the outer periphery of a welded tube 2. The same number of nozzles 11 are provided at the angle positin corresponding to respective temp. sensor 8 to the annular member 10. A controllng device 14 is provided between these temp. measuring device 5 and a cooling device 6, and the cooling water volume according to the temp. distribution in peripheral direction of the welded pipe is released from a selected nozzle 11 by receiving a signal from the thermometer 9. In a drawing stage the device 5 detects the position of the high temp. forge welding part 4 and a cooling water is released by opening the nozzle 11 corresponding to the forge welding part 4. The temp. distribution in peripheral direction of the welded pipe 2 is equalized and the generation of thickness deviation is always prevented by said method.