PURPOSE: To provide a manufacturing method of a polygon mirror and a device therefor in a compact form to cut and divide the mirror surface of the polygon mirror, with which the cycle time for attaching or detaching a workpiece is reduced while two workpieces can be cut simultaneously, and with which high working precision is achieved for the mirror surface.

CONSTITUTION: Workpieces 1a, 1b are mounted on the both end parts of a workpiece holding jig 13 opposed to the rotation trace circle of fly cut tools 23, 24, so that the distance between the workpieces is almost equal to the diameter of the locus circle. Rough cutting and finish cutting are carried out in the both processes of reciprocating movement of the workpiece, while minimizing the distance of the reciprocating movement, and since the division on both ends and attachment and detachment of the workpiece when the overall mirror surface is finished, are carried out for the two workpieces simultaneously in each process, the overall work efficiency becomes high. Since the precision of the parts of a divider is high in the method employed, the precision of the mirror surface of a product becomes also high.