A copying apparatus comprises original sheet holder provided with a plane on which an original sheet is to be mounted, exposure system provided with a lens to guide light rays reflected from an impression indicated on the original sheet mounted on the holder to cause the original sheet impression to be focused on a photosensitive layer. A detector for detecting data on an average original density of the original sheet impression denoted by the light rays is set between the photosensitive layer and lens to receive part of the light rays reflected from the original sheet, on a line extending from the center of the lens to the center of that portion of the photosensitive layer which corresponds to the smallest original sheet among those which are to be mounted on the original sheet holder.