A device for detecting a defect on the end face of a glass sheet is provided with: an image pick-up device having at least two CCD cameras for image-picking up the end face of the glass sheet from outside the glass sheet and in two directions diagonal to both front and back surfaces thereof; an illuminating device having a nearly C-shaped ring illumination capable of applying illuminating light in its center axis direction and having an opening slit; and an image processing device for processing the image signals acquired from the CCD cameras to determine the quality of the end face. The glass sheet is loosely inserted in the opening slit so that the end face agrees with the center axis of the ring illumination; the illuminating light is applied to the end face; and the end face is image-picked up by the image pick-up device. The image signals thus acquired are processed by the image processing device, thereby detecting the presence/absence of the defect on the end face.