PURPOSE: To execute copying of a good quality by detecting a reflecting density of a ground of a copying original by preceding a copying process, and executing by a constant optimum exposure in the course of an operation of the copying process.

CONSTITUTION: A sensor 19 converts a reflecting density of a ground of a copying original 12 to an electric signal, varies impressed voltage of an exposure lamp 13 through an operation processing circuit 23 and a controlling circuit 24, turns on the exposure lamp 13 by a correct exposure quantity, and when the first optical part 20 starts its movement to a point B from a point A, the exposure process is started, and the copying process is completecd through development, transfer and fixing processes successively. In the course of operation of the copying process, the optical part 20 executes a reciprocating motion between the point A and the point B, but since the sensor 19 and a sensor holding member 21 are moved to a point D, there is no hindrance in the operation of the first optical part 20. When the copying process is ended, the optical part 20 stops at the point A, the post-processing process is executed for several seconds, and thereafter, all the processes of a copying machine are ended and the operation is completed, but within the execution time of the post-processing process, a sensor holding member driving mechanism 22 is operated in the opposite direction, and the sensor 19 is reset to the point A from the point D.