PURPOSE: To obtain a desired ultrasonic focus depth with respect to the flaw detection range of the surface part of an object to be inspected, by allowing a longitudinal ultrasonic wave to be incident from an oblique angle probe for receiving a longitudinal wave and receiving the reflected scattered wave due to the flaw in the object to be inspected by a vertical probe for receiving a longitudinal wave.

CONSTITUTION: An ultrasonic wave is incident on an object 3 to be inspected from a transmission ultrasonic probe 1 and the reflected scattered wave from the flaw 4 in the object 3 to be inspected is received by a receiving ultrasonic probe 2. Herein, on an oval shape spherical surface wherein both probes 1, 2 are focal points, the line on the oval shape comes to an equal beam distance line 9. Therefore, the distance δ between the probes is fixed to perform flaw detection and, when a flaw echo is obtained and a beam distance is calculated, it can be estimated that the flaw 4 is present on the oval shape. Further, when the distance δ between the probes is changed or a pair of the probes 1, 2 are moved, a separate oval shape is successively determined and the intersecting points of all oval shapes are determined as flaw positions. In this constitution, if a longitudinal wave oblique angle probe is used as the probe 1 and a vertical probe is used as the probe 2, because the vertical probe receives only a vertical direction, a flaw detection is determined even if a separately determined oval-shape is not calculated.