A defect inspection apparatus comprising: an ultrasonic wave probe; a probe holder (la) adapted to grip the ultrasonic wave probe; a scanning unit connection jig rotatably connected via a connecting screw to the probe holder, and adapted to rotate as a single body the probe holder together with the connecting screw; an ultrasonic probe drive device connected to the scanning unit connection jig, and a control unit adapted to control said drive device so as to move the ultrasonic wave probe along a surface of an inspection material and also to lower the ultrasonic wave probe towards the surface of the inspection material so that the ultrasonic wave probe comes into contact with the surface of the inspection material at each inspection position thereon; an ultrasonic wave transmitting and receiving device adapted to irradiate ultrasonic waves via the ultrasonic wave probe onto the surface of the inspection material on which an oil having a density of 1 g/cm 3 or less and a kinematic viscosity of 100 mm 2 /s or less has been provided, and to also receive as noise signals ultrasonic waves that have been scattered by defects present in the interior of the inspection material; a frequency spectrum calculation device adapted to perform time division on the noise signals so as to divide them into time widths (Tg) that correspond to positions in the depth direction of the inspection material, and to calculate a frequency spectrum for each one of the time-divided noise signals; and a defect distribution detection device that, based on the frequency spectra, is adapted to calculate values showing a level of defect progression corresponding to a position in the thickness direction of the inspection material; wherein the control unit is adapted to control the drive device so that: after the ultrasonic wave probe has come into contact with the surface of the inspection material via the oil, force is applied in a direction in which a gap between the probe holder (la) and the scanning unit connection jig becomes smaller, thereby causing the connecting screw to rotate together with the probe holder so that the ultrasonic probe is rotated in an in-plane direction of the surface of the inspection material. an ultrasonic wave probe; a probe holder (la) adapted to grip the ultrasonic wave probe; a scanning unit connection jig rotatably connected via a connecting screw to the probe holder, and adapted to rotate as a single body the probe holder together with the connecting screw; an ultrasonic probe drive device connected to the scanning unit connection jig, and a control unit adapted to control said drive device so as to move the ultrasonic wave probe along a surface of an inspection material and also to lower the ultrasonic wave probe towards the surface of the inspection material so that the ultrasonic wave probe comes into contact with the surface of the inspection material at each inspection position thereon; an ultrasonic wave transmitting and receiving device adapted to irradiate ultrasonic waves via the ultrasonic wave probe onto the surface of the inspection material on which an oil having a density of 1 g/cm 3 or less and a kinematic viscosity of 100 mm 2 /s or less has been provided, and to also receive as noise signals ultrasonic waves that have been scattered by defects present in the interior of the inspection material; a frequency spectrum calculation device adapted to perform time division on the noise signals so as to divide them into time widths (Tg) that correspond to positions in the depth direction of the inspection material, and to calculate a frequency spectrum for each one of the time-divided noise signals; and a defect distribution detection device that, based on the frequency spectra, is adapted to calculate values showing a level of defect progression corresponding to a position in the thickness direction of the inspection material; wherein the control unit is adapted to control the drive device so that: after the ultrasonic wave probe has come into contact with the surface of the inspection material via the oil, force is applied in a direction in which a gap between the probe holder (la) and the scanning unit connection jig becomes smaller, thereby causing the connecting screw to rotate together with the probe holder so that the ultrasonic probe is rotated in an in-plane direction of the surface of the inspection material.