To increase the area of an optical device.

A diffraction optical device having eight steps of BO form is formed in a fanlike block 22 cut along the diameter of the original circle into four parts on a quartz substrate 21. By aligning the fanlike blocks 22 while observing the pattern on the borders of the fanlike blocks 22 adjacent to each other, the side faces of the fanlike blocks 22 are joined by optical contact. Then the peripheral part is fixed by using a cell 27 so that a diffraction optical device 28 is obtd. from the four blocks.