A high-grade, high-efficiency processing method was established for nonlinear chalcone crystal. substrates for use in frequency converters, based on the mechanical and chemical characteristics of the organic crystal. First, the basic processing characteristics of the polishing slurry made from polyethylene glycol (PEG) solution and super-micro particles for the application to chalcone crystals were fully grasped. Subsequently, a "rotating annulus-feed processing method," in which the;said slurry is contained in a specific container, was devised, and its superiority in the application was confirmed. In accordance with this outcome, simultaneous, double-side polishing equipment was designed based on a bowl-feed formula and manufactured on a trial basis. This equipment made it possible to polish simultaneously double sides of a chalcone crystal efficiently producing a mirror-surface roughness R(max) of below 3 nm (at the stock removal rate of 2 mu m/min). It also has been confirmed that the quality of workpieces processed by this method does not change even with the passage of time. As a result, second-harmonic generation (SHG) became possible.