A plurality of platy optical elements, capable of downsizing an optical device by using an octagonal shape and being positively fixed to the inner bore of a permanent magnet by brazing, are efficiently cut out of a large-area platy optical material. Circular, round or oval, punched holes (11, 12...) with their centers located at the crossing points (P1, P2...) of dividing lines (C1..., C2...) dividing a platy optical material (10) laterally and longitudinally are formed in a plate surface, the plate surface of the optical material (10) provided with the holes (11, 12...) is cut laterally and longitudinally, and a plurality of platy optical elements each providing an almost octagonal outer shape formed by four each of alternately continuous linear sides and arcuate sides are obtained.