PURPOSE: To permit safe correction in a short time by providing a supersonic probe in a pipe and a grinding wheel opposed thereto and movable axially outside the pipe and determining stepwise correction amount to carry out stationary grinding and rotational grinding with computer control by wall thickness signal.

CONSTITUTION: Correction frequency, desired wall thickness and length to be corrected at each stage are sent to the input of a computer 3 by a typewriter 26. Initially as a pipe 1 is stationary, a grinding unit 8 is advanced by a cylinder 19 and controlled through the computer 3 by the measured wall thickness signal from a supersonic probe unit 7 to grind the pipe to the desired wall thickness. Next, the pipe 1 is rotated by a drive unit 4 and interlocked with the grinding wheel unit 8 and probe unit 7 which are moved along a slide rail 9 the correction length by a feed shaft 10 under the control of computer with the output of a measuring machine so that the outer periphery is ground spirally to complete the first pass correction. Similarly, the pipe is stepwise correctively ground to the desired correction wall thickness.