The invention can easily and accurately detect a position, a depth, or a height of a damaged portion such as a cracked surface and a corroded portion and a micro cracking near the surface layer of a thick object. An ultrasonic wave (16) is introduced from an oblique direction to an object (6) to be inspected so as to detect diffractive waves (17) generated at the end of a damaged portion (20) in the object (6), above the damaged portion. The position of the end of the damaged portion (20) from a surface (13) of the object (6) is obtained by using the trigonometric measurement based on an all-beam path (Wt) of a component (18) propagating directly above the damaged portion (20) among the diffractive waves, from the incident position via the damaged portion, and an interval (S) between the incident position and the detection position of the ultrasonic wave, or a propagation time difference (tt - ts) between the reception of a surface wave (15) reflecting these relationships and a diffractive wave (18) propagating directly above the damaged portion.