here ?? is the wavelength of a light beam to be used and n is a positive integer; each of said first phase pits having an optical depth of (2n???1???4m)??/8 where m is an integer not less than 0; each of said second phase pits having an optical depth of (2n+1???4s)??/8 where s is an integer not less than 0; said n, m, and s being related so as to satisfy conditions of 2n???1???4m>0 and 2n+1???4s>0