u03b8 are calculated based on central thicknesses of the right and left lenses s5 then the central thicknesses satisfying a minimum et condition for every lens power of the right and left lenses are determined s6 next whether the bevel edge is provided or not is determined s7 when providing the bevel edge the central thicknesses satisfying the minimum et condition is determined in accordance with step s6 and the operation proceeds to the unit for adjusting the balance between the right and left lens thicknesses whereas if no bevel edge is provided whether the frame is the two point frame or not is determined s8 if not the two point frame it follows that a groove is formed in a lens edge surface a minimum et e g 2 mm at this time is specified and whether an et is larger than this specified value or not is determined s9 if the et is larger than the specified value the central thickness is set as the minimum et condition is satisfied and the operation proceeds to a processing step of a thickness balance adjustment whereas if the et is smaller than the specified value the central thickness is determined according to the specified value e g 2 mm.