PURPOSE: To detect a flaw at a high accuracy by making a fluorescent magnetic powder in an inspection liquid adsorbed by a false flaw formed with a leakage flux generator to detect the intensity of light emitted from the fluorescent magnetic powder adsorbed.

CONSTITUTION: An inspection liquid 2 mixed with a fluorescent magnetic powder is stored into an inspection liquid tank 1 and is sprayed fully to a steel material 7 to be measured with a pump 4 from the nozzle 6 of a sprayer 5. At this point, the steel material 7 is magnetized with a magnetizer and a larger amount of the fluorescent magnetic powder is adsorbed at the spot having a flaw than those at other parts. The excess inspection liquid is enters a tray 8 to return to the tank 1. On the other hand, a part of the inspection liquid is fed to a characteristic measuring apparatus 11 passing through a valve 10 from a pump 4' to measure the characteristic thereof. The steel material 7 is carried to a flaw detector 41 and a marking is done at the position accurately corresponding to the flaw detected on the surface of the steel material 7 with a marking device 49 to indicate the position where a flaw exists.