PURPOSE: To decide whether a defect of a thin cylinder exists or not, on the same pipe working machine, almost simultaneously with or immediately after forming of the thin cylinder, by placing a flaw detecting head on the pipe working machine for forming a thick cylinder to a thin cylinder.

CONSTITUTION: To a thick cylindrical blank pipe 3 fitted into a rotating mandrel 1, a rotating roller 4 of the outside circumference of the mandrel 1 is pressed the base pipe 3 is cold-rolled in the axial direction, and a thin cylinder 2 is manufactured. When the cylinder 2 is magnetized by high frequency from for instance, an overcurrent flaw detecting head 5 on this pipe working machine, an overcurrent is generated on the surface of the cylinder 2, impedance of a coil in the head 5 is varied by the time when there is no defect or there is a defect. Accordingly, its difference is fetched and amplified electrically, and when it is detected by a comparator that it has exceeded some extent, an alarm is raised, or a making device is interlocked, and a position of a defect can be displayed on the cylinder 2.