PURPOSE: To calibrate the measuring accuracy of a measuring device quickly and accurately, by providing a test piece, wherein a reference surface to which a steel pipe end detecting probe is contacted and a reference surface to which a coupling end detecting probe is contacted are made to be a unitary body through a reference coupling length, so that the test piece can be arranged at a measuring position.

CONSTITUTION: When a coupling length measuring device is calibrated, a test piece 11 is arranged at a measuring position shown by a solid line by a switching actuator 12. A probe 6 is moved to the contact point with a reference surface 11A from a base point, and the moving distance l1 is detected. A probe 9 is moved to the contact point with a reference surface 11b from a base point, and the moving distance l2 is detected. Based on the length L between the base points of the probes 6 and 9, a reference coupling length l is computed by an expression l=L-(l1+l2), and compared with a reference value l0. Thus the measuring accuracy can be calibrated quickly and accurately.