PURPOSE: To eliminate severe limitation with respect to the straightnees of a material to be inspected, by a method wherein a rotary probe holder is formed into a double cylinder consisting an outer cylinder and an inner cylinder and the inner cylinder is concentrically held by buoyancy generated with respect to centrifugal force of the inner cylinder.

CONSTITUTION: Water suplied to the outer cylinder 31 of a rotary probe holder 14 is guided to a water channel space 21' formed by the inner wall of the outer cylinder 31, the outer wall of an inner cylinder 32 and bellows 33, 34 and passed through the space formed between the protrusion 39a of an attaching spacer 39 and a mounting hole 41 to fill a void 24 with catalytic medium water. Ultrasonic beam emitted from a probe 20 by said catalytic medium water is injected in a material 6 to be inspected while the reflected echo is received by the probe. In this case, it is necessary that the inner cylinder 32 is rotated at a concentric position with respect to the outer cylinder 31 when the material to be inspected is advanced into the probe holder. Therefore, the mass of the inner cylinder is made smaller than that of water excluded by a total volume and the buoyancy of the inner cylinder is utilized with respect to centrifugal force generated when the probe holder is rotated at a high speed to hold the inner cylinder concentrically. Restriction due to the bending of the material to be inspected is eliminated and flaw detection can be performed at a high speed in high efficiency.