PURPOSE: To harden metallic pipes homogenously without giving strains such as bending deformation and roundness defects by setting the velocities of flow of cooling liquid at specific velocities or above in the stage of hardening the metallic pipes by applying the flow of the cooling fluid along the axial direction of the pipes on the inside and outside circumferential sides of the pipes.

CONSTITUTION: A cylindrical body 1 which is made freely openable and closable vertically or laterally is disposed approximately horizontally in a cooling liquid tank 3 contg. cooling liquid 2 and a red hot metallic pipe 4 is inserted into the body 1. On the other hand, flow rate control valves 10, 11 are provided on the respective mid-ways of a piping 8 running from a pump 7 to an outer nozzle 5 and a piping 9 running to an inner nozzle 6. The flow rates on the outside and inside circumferential sides of the pipe 4 are set in the following way by controlling these valves 10, 11. The velocities of flow of the cooling liquid in the position approximately 10mm from the outside circumferential surface of the pipe 4 and the position approximately 10mm from the inside circumferential surface are set respectively at ≥1.5m/sec, and the pipe is hardened under these velocities of flow.