Improvements in a magnification varying device for electrophotographic copying machine of the mirror scan type to eliminate the drawbacks to be encountered and resulting in degraded copy images when the magnification varying range is increased, such as objectionable operative connection between a projection lens and a deflection mirror, and leakage of undesired light between the lens and a light blocking member around the path of travel of the lens. The present device includes an eccentric cam having the largest radius at a position for giving an equal magnification to assure smooth transmission of drive force to the deflection mirror, and a second light blocking member provided at each side of the lens and opposed to the mirror for preventing the objectionable light not blocked by the above blocking member from reaching the mirror when the lens is qat a position for a reduced magnification.