PURPOSE: To deal with an object even when it is a negative original or a positive original and to enable trimming of the object by simple and inexpensive cheap process by providing a control board that shields light from the object and a light source for trimming a positive original.

CONSTITUTION: When the object is a positive original and trimming is desired, a line light source 5 is lit up, and a control board 1 is installed at a position B for the part in the direction of flow of copying of the object 8. For the part at right angles to the direction of flow, control boards 21, 22 are shifted to specified positions in the directions C, C' respectively, and a slit 40 is shaded by the control boards 21, 22, and trimming is made by reflected light of the light source 5. When the object is a negative original and trimming is desired, the slit 40 is shaped by the boards 21, 22 as in the case of a positive original, and image forming and trimming is made by applying bias voltage. Thus, by providing control boards 21, 22 and the light source 5, trimming can be performed simply and inexpensively irrespective of kind of original.