PURPOSE: To suppress the occurrence of a flaw-detecting unabled region in the proximity of opposite ends of a material to be measured, by a method wherein, when the forward end and the rear end of the material to be measured pass through a position of or in the proximity of a flaw finding shoe, they are sharply reduced in its a pass speed for a given period of time.

CONSTITUTION: If a steel pipe material 1 to be measured is conveyed into a flaw detector 2, the forward end of the mateiral 1 to be measured shields a photoelectric tube 6. This enables a length measuring device 8 to make contact with it, and a measuring length is counted by means of a signal from the photoelectric tube 7. At the point of time when only a distance between the device 8 and flaw-detecting shoes 3, 3' is measured, the shoes 3, 3' are brought into contact with the material 1 to be measured. At this point of time, deceleration of a conveying speed of a conveyor has already been completed to a very slow speed range from a high speed range. After the very slow speed range is held for a given period of time, an acceleration starts in a similar pattern to that of the deceleration, and it returns back to the high speed range. As described above, in case the forward end and the rear end of the material to be measured pass through the position of or in the proximity of the flaw-detecting shoes so as to match a timing when the flaw detecting shoes make contact with and part from the material to be measured, the conveyor is sharply reduced in its speed for a given period of time, which results in suppressing the occurrence of a flaw-detecting unabled region.