PURPOSE: To make it possible to detect the defect of a welded part accurately, by detecting a fused tip position by using a vertical prove, detecting the echo of the defect by a slant angle probe, and handling only the echo in the welded part as an effective signal.

CONSTITUTION: A probe holder 7 is scanned on a web 2 through a contact medium 6. A slant probe 5 and a vertical probe 8 are mounted on the holder 7. A fixing rod 9 extending into the vertical direction is attached to the holder 7. The probes 5 and 8 are connected to ultrasonic wave flow detectors 10 and 10', respectively. The echo of a defect is detected by the slant probe 5. On the echo in the welded part is handled as an effective signal. Thus the defect in the welded part can be detected accurately.