Abstract: An optical device in a variable magnification electrostatic copying apparatus includes a first reflecting mirror assembly mounted for reciprocal movement, a second reflecting mirror assembly mounted for reciprocal movement substantially parallel to the reciprocating direction of the first reflecting mirror assembly, and a speed-reduction interlocking mechanism for moving the second reflecting mirror assembly at a speed equal to one-half of the moving speed of the first reflecting mirror assembly, upon movement of the first reflecting mirror assembly. The optical device further includes a magnification setting mechanism for changing the relative position of the second reflecting mirror assembly with respect to the first reflecting mirror assembly according to a desired copying magnification.