PURPOSE: To maintain a voltage constant and to form a good hardened layer to a uniform depth even when the inside surface of a steel pipe is rugged by rotating the pipe at a low speed and moving the preceding end of a torch forward and backward while maintaining a specified distance from the inside surface of the pipe.

CONSTITUTION: A plasma jet torch 2 is located in the end part of a steel pipe 1 so as to face the inside surface thereof. While the torch 2 is moved in the axial direction of the pipe 1, which is kept rotated at a low speed, by a weaving device 6, plasma jets are ejected from the torch. A voltage is compared with a reference voltage and is differentially amplified by an automatic correcting device 7 for vertical positions by making use of the change in the voltage when the distance from the preceding end of the torch 2 and the inside surface of the pipe changes. In accordance with the compared value, the supporting arm 21 for the torch 2 is moved vertically by means of a servocontrol unit 71 along the sliding base on the device 6. As a result, the distance between the plasma jet injecting position on the inside surface of the pipe 1 and the torch 2 is kept constant.