PURPOSE: To make it possible to measure at which position a roll shaft center is to a mill center as reference.

CONSTITUTION: In a centering measuring instrument of a piercing mill to measure the position to the mill center of the roll 1 of the piercing mill, a three dimensional measuring instrument 6 consisting of a laser beam emitting device 5 agreeing nearly to the mill center provided on the inlet side or the outlet side of the piercing mill, a sensor head to receive laser beam, a non-contact displacement gage which can measure a roll gap in non-contact and a linear encoder to measure a moving amount of the non-contact displacement gage moving in the mill center direction and in the vertical direction is provided. Further, this device contains an arithmetic display device to operate and display a deviation between the mill center and the roll center in the horizontal direction, an angular deviation of a crossed axes angle and a deviation between a mill center and a roll center in the vertical direction and an angular deviation of an inclined angle in accordance with the central coordinate of laser beam measured by a three dimensional measuring instrument, a distance as far as the roll measured by the non-contact displacement gage and a moving amount in the mill center direction and the vertical direction of the non-contact displacement gage measured by a linear encoder.