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.