An object of the present invention is to quantitatively evaluate a distribution of defects which are generated within an inspection material. In order to achieve this object, the present invention provides a defect inspection apparatus comprising: an ultrasonic wave probe; an ultrasonic wave transmitting and receiving device that irradiates ultrasonic waves via the ultrasonic wave probe onto a surface of an inspection material on which a predetermined propagation medium has been provided, and that also receives as noise signals ultrasonic waves that have been scattered by defects present in the interior of the inspection material; a frequency spectrum calculation device that performs time division on the noise signals so as to divide them into time widths that correspond to positions in the depth direction of the inspection material, and calculates a frequency spectrum for each one of the time-divided noise signals; and a defect distribution detection device that, based on the frequency spectrums, calculates values showing a level of defect progression corresponding to a position in the thickness direction of the inspection material.