A dielectric substrate having a refractive index n.sub.1 is covered with
an amorphous dielectric medium having a refractive index n.sub.2 larger
than the refractive index n.sub.1, and a rectangular grating is formed in
the amorphous dielectric medium through an etching process. When
microcrystal is formed in the amorphous dielectric medium through a
thermal treatment to increase the refractive index n.sub.2, a magnitude
.DELTA.n of structure double refraction increases accordingly. Hence, a
phase refraction .DELTA..PHI. of an irregular grating pattern in the
rectangular grating becomes large without increasing a depth D of each
trench of the rectangular grating, and thus it is possible to obtain an
optical retardation plate having a fine periodic structure and having a
desired plate retardation.