PURPOSE: To simultaneously measure not only the roll center of bar steadyer but the parallelism to the mill center.

CONSTITUTION: This device consists of a laser beam emitting device irradiating in the opposite direction with laser beam which is provided on one of an inlet side or an outlet side of the bar steadyer 3 of a perforating mill, a diffusion plate 19 which can turn round the axis of a dummy tube 19 supported by three rolls of the bar steadyer and is arranged movably in parallel with the axial direction of the dummy tube 18 and a beam receiving device made of a CCD camera 22. Further, it contains an arithmetic display device which requires the difference between the axis of the dummy tube 18 and the laser beam obtained based on a beam received result of the beam receiving device and which calculates the parallelism between the pass center of the mill and the axis of the bar steadyer. In this way, the axis of the bar steadyer can be measured with high accuracy and in a short time and dimensional fault such a thickness deviation of a product generated by deviated alignment can be reduced.