Document detection device and a copying machine provided with such a device

After placing an original document on a document platen of a copying apparatus main body, when a detection switch detects a predetermined angle of theta of a document cover in the course of closing the document cover, a light-emitting device of a distance-measuring sensor is driven. Light released from the light-emitting device is projected onto the original document or the document cover, and the reflected light is received by a light-receiving device. A signal corresponding to the light-receipt point of the light-receiving device is released, and a measurement is made to find the distance from the sensor to the original document or to the document cover based on the signal. A discrimination is made as to the presence or absence of original document based on the measured distance, and copying control suitable for the result of the discrimination is carried out. At this time, if external light or other light is received by the light-receiving device, the output of the light-receiving device exceeds a predetermined range. This state is regarded as no original document is located above the related distance-measuring sensor, and the corresponding process is carried out.