PURPOSE: To provide a titled device which forms a satisfactory hardened layer layer to a uniform depth, by rotating a steel pipe at a low speed and providing a means for feeding a wire while moving a plasma torch back and forth in the pipe with a position corrector and a weaving device.

CONSTITUTION: A plasma jet torch 2 is placed to face the inside surface at one end of a steel pipe 1, and while the pipe 1 is rotated at a low speed, the torch 2 is kept moved in an axial direction of the pipe 1 and a plasma jet is injected therefrom. At the same time, a wire 9 is fed to the torch 2 by a wire feeder 92 and the molten metal of the wire 9 is injected to the inside surface of the pipe 1 while the inside surface of the pipe 1 and the wire 9 are simultaneously melted. The weaving of the torch 2 by a weaving device 6 is increased in speed and the inside surface of the pipe 1 is remelted by the plasma jet drawing a saw tooth- like line so that the molten metal on the inside surface of the pipe 1 and the molten metal of the wire 9 are fused together. The hardened layer having the increased hardness after solidification is formed to a uniform depth by the above- mentioned mechanism.