PURPOSE: To simplify flaw detection by a method wherein an apparatus is once installed at an apparatus installation position on a rail mounted along each of a longitudinal welded part and a peripherally welded part of a vessel.

CONSTITUTION: Flaw detection of a longitudinal welded part of a reactor pressure vessel 13 is started after a device 17 has been moved to a part to be inspected with a longitudinal rail scanning mechanism 2 of the device 17 driven on a longitudinal rail 1. Flaw detection of a peripherally welded part of the reactor pressure vessel 13 includes attaching the device 17 to the longitudinal rail 1, then moving it to a cross point of the longitudinal rail 1 and a peripheral rail 9, operating a peripheral rail scanning mechanism 5 with a controller 19 driven, moving it from a device mounting rail onto the peripheral rail 9 and moving the peripheral rail scanning mechanism 5 to a part to be inspected for flaw.