PURPOSE: To change an optical path with the movement of a lens, to obtain two kinds of reduction optical magnification, to cope with the size of the width of an original and to always obtain a large number of picture elements.

CONSTITUTION: This mechanism is provided with a fluorescent lamp tube 1, a first mirror 5 reflecting reflected light from an original 2, a second mirror 6, a lens 8 integrally supported by a third mirror 7 and a line sensor 9. Two mirrors 11 and 12 are newly provided on the upper and lower parts of a carriage 10 so that another optical path different from the optical path obtained at the time of no movement of the lens 8 can be obtained when the lens 8 integrally supported by the third mirror 7 is moved along a feeding screw 13 by the driving force of a motor 14.