In a method for processing a plate object, etching is performed for a flat plate object by a predetermined etching method and the shape of the plate object after the etching is grasped in advance. In a grinding step, the plate object(1) is ground into a grinding-finished shape that is a non-flat shape obtained by inverting the shape of the plate object(1) after the etching to the reverse shape. When subsequent etching by the predetermined etching method is performed for a grinding-target surface, the plate object(1) is formed into a flat shape with a uniform thickness.