PURPOSE: To move an ultrasonic flaw-detecting element spirally in an accurate and smooth way and thereby to perform reliable flaw detection without any impediment, by moving the flaw-detecting element in the axial direction with rotation by a rotating motor, an axial feed motor and an axial feed nut and also by disposing an axial feed shaft thrust bearing in a prescribed part.

CONSTITUTION: An ultrasonic flaw-detecting element 1 of an ultrasonic flaw detector inserted in to a boiler tube or the like under the water pressure or the like is rotated by a motor 3 for rotation and moved in the axial direction through the intermediary of an axial feed motor 6 and an axial feed nut 5. Thereby the element is moved spirally in a smooth and accurate way, so that the reliable ultrasonic flaw detection of a welded part or the like of the boiler tube be executed. Besides, an axial feed thrust bearing 7 supporting a part of connection of the fore end of a flexible shaft 8 connected to the motor 6 with the rear end of a flexible shaft 9 connected to the nut 5 is provided between the nut 5 and the motor 5. Therefore an excessive tension in the axial direction due to a water pressure applied when the flaw detector is inserted in not transmitted to the motor 6, and consequently the breakdown of the motor 6 is prevented.