a main body having supporting mounts; and

a unit chassis including optical instruments for image formation being mounted thereon, and said unit chassis being fixedly supported on said supporting mounts;

said unit chassis having reinforcing areas reinforced by a reinforcing structure, said optical instruments being mounted in said reinforcing areas;

said unit chassis having three main fixing members formed connected to respective ones of said reinforcing areas so as to be reinforced by said reinforcing structure, said three main fixing members being formed one of within said respective reinforcing areas or contiguous to said reinforcing structure, said three main fixing members being respectively fixedly fastened to said supporting mounts;

said unit chassis having at least one subsidiary fixing member connected to said reinforcing areas by a buffer area and a flexible member having flexibility greater than connections of said three main fixing members to said reinforcing areas, and said subsidiary fixing member being fixedly supported by a respective one of said supporting mounts and said buffer area deflecting such that subsidiary fixing member is deflective from a position relative said three main fixing members prior to mounting of said unit chassis to said supporting mounts and such that said deflecting reduces vibration of said unit chassis, said reinforcing areas having sufficient rigidity such that said reinforcing areas remain substantially free of warping when said subsidiary fixing member is deflected relative thereto thereby maintaining positional accuracy of said optical instruments mounted in said reinforcing areas;

said three main fixing members and said subsidiary fixing member being respectively formed in a plurality of places that define a polygon having at least four angles in edges or in vicinity thereof of said unit chassis.