Abstract: Ultrasonic waves are used to detect the size and the depth of an internal defect S in the face portion 2 of a panel 1. Preferably, the ultrasonic wave is transmitted from the outer surface 2a side of the face portion 2 toward the inner surface 2b to receive a reflected wave reflected on the inner surface 2b side, thereby detecting the size of the internal defect S and the distance of the internal defect S from the inner surface 2a. Upon transmission and reception of the ultrasonic wave, the ultrasonic wave is allowed to propagate through a non-compressive fluid as a medium outside the face portion 2.