PURPOSE: To obtain a copy image of uniform density corresponding to variation in copy magnification and fast copying by controlling the light quantity of a lamp according to a part where original density is low even when an exposure lamp has a response time.

CONSTITUTION: When the density of an original 1 is low, the quantity of reflected light increases and the output of a detecting element 9 is large; and the output of a reference voltage generating circuit 23 is constant and the input difference to a comparing circuit 21 becomes less, so the input voltage to the exposure lamp 3 drops, so that the quantity of light decreases. When the original density is high, on the other hand, the input voltage to the lamp 3 rises to increase the quantity of light, and the exposure amount of a photosensitive drum 10 is set to a constant value proportional to the output of the circuit 23 regardless of variation in the original density. Further, the original density is read previously, so when the density measurement part of the original is exposed through a slit, control is carried out to the quantity of light corresponding to the preread density after the transient period of the lamp 3, obtaining a copy with uniform density. A delay circuit 22 corresponds to plural stepless variation and generates signals proportional to respective scanning speeds, so that the smaller the copy magnification, the shorter the delay time.