1. A device for detecting a defect on an end face of a glass sheet, the device comprising: an image pick-up device including 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 of the end face of the glass sheet; an illuminating device including a C-shaped ring illumination which is capable of applying illuminating light in its center axis direction and has a slit; and an image processing device adapted to process an image signals acquired from the CCD cameras and determine a quality of the end face, wherein the glass sheet is loosely inserted in the opening slit so that the end face agrees with a center axis of the ring illumination; the illuminating device applies the illuminating light to the end face; and the image signals acquired from image pick-up of the end face by the image pick-up device are processed by the image processing device to detect the defect on the end face.