PURPOSE: To uniform the distribution of the quantity of light by adjusting the quantity of transmitted light roughly based on the magnification of projection by a control means and adjusting it finely based on the signal detected by a photodetecting means.

CONSTITUTION: In case a picture on a microfilm F is read, the projection magnification of a zoom lens 5 is detected by a slide volume VR as a magnification detecting means, and the quantity of transmitted light of a dimming glass 60 is adjusted roughly by this magnification detection signal. Thereafter, the illuminating light passing the dimming glass 60 is detected by a light quantity detecting sensor 14 as the photodetecting means, and the quantity of transmitted light is adjusted finely to uniform the distribution of the quantity of light. A dimming glass selecting and driving circuit 82 is connected to the dimming glass 60 arranged in an exposure slit 18 to vary the quantity of transmitted light partially in the lengthwise direction of the exposure slit 18. A means which controls the voltage of the dimming glass 60 uses the center part in the lengthwise direction of the slit as the reference position and detects the distribution of the quantity of passing light in the lengthwise direction of the slit as an electric signal, and the quantity of light transmitted through the slit is controlled by the voltage so that the difference between maximum and minimum detected values is reduced.