PURPOSE: To measure the pass center of bar steadyer with high accuracy and in a short time.

CONSTITUTION: This device consists of a laser beam emitting device 2 to irradiate laser beam which is provided on the inlet side of the bar steadyer 3 of a piercing mill 1, the sensor head 6 of a beam receiving position detector provided facing the laser beam emitting device 2 and in agreement with the reference pass center of the bar steadyer on the outlet side and a dummy tube 9 held in each bar steadyer fixed to the tip of the mandrel bar 10 so that the sensor head of the beam receiving position detector to receive built-in laser beam and the center of the mandrel bar 10 are positioned on a concentric circle. Further, an arithmetic display device to require the difference between the axis of the mandrel bar 10 obtained from the beam receiving result of the beam receiving detector and the reference pass center of the laser beam is contained. In this way, dimensional fault such s thickness deviation of a product generated by deviated alignment can be reduced.