A rotary polygonal mirror has a base member with a portion of a polygonal cross-section constructed of such a relatively light-weight material as can easily be formed into a relatively complicated configuration. The polygonal portion includes a reflective mirror face formed on one or more sidewalls thereof. A method of making such rotary polygonal mirror includes the step of molding the base member with or without subjecting one or more sidewalls of the polygonal portion to a precision cutting to form a reflective mirror face thereon.