PURPOSE: To apply ultrasonic beam to a welding line in perpendicular or horizontal state and improve the inspection effect, by a method wherein a probe can be moved in perpendicular or parallel direction to both circumferential and axial welding lines.

CONSTITUTION: When a flaw of a welding line 23 in circumferential direction is inspected (welding lines 37, 38 in similar manner), a drive device 25 is stopped just above the welding line 23 along a track 21, and a ring 33 is moved in circumferential direction and a probe 28 is moved along an arm 29. On the contrary, when a flaw of a welding line 24 in axial direction is inspected, the probe 28 is stopped near the center of the arm 29, and the ring 33 is moved in the circumferential direction and the drive device 25 is moved along the track 21. As a result, the ultrasonic beam can be applied to both the circumferential welding line 23 and the axial welding line 24 in perpendicular or horizontal state. In this constitution, the inspecting performance is improved and the beam travel can be calculated without consideration of angle, thereby signal processing for position detection is simplified and the on-line inspection is enabled.