PURPOSE: To easily perform the setting of the origin of a probe even when the installation position of a flaw detection apparatus was shifted, by setting the origin to the welded part of a pipe body.

CONSTITUTION: A flaw detection apparatus is constituted of a ring shaped guide rail 2, a drive apparatus 3 moving the peripheral direction along the rail 2, the arm 4 and ball screw 8 supported by the drive apparatus 3, the ultrasonic probe 5 attached to the arm 4 and the ball screw 8 and a control apparatus 10 and attached to a tubular body 1 to be tested. The control apparatus sets the graduations of a display part 1 to 0 (origin) when a welding line 7 coincided with the center line 6 of the probe 5. By this method, even when the mount position of the guide rail 2 was shifted, the position of the probe 5 can be set easily and accurately.