To provide a processing method which can form a processed surface of high flatness without having an affected layer by processing a surface to be processed of a workpiece.

The processing method includes: a workpiece disposing step for disposing a workpiece 20 having a processing surface 20a in a processing solution 30; a photocatalyst film disposing step for disposing a photocatalyst film 12 in the processing solution 30 opposite the processing surface 20a; an active species creating step for irradiating the photocatalyst film 12 with light to generate active species 40 from the processing solution 30 by a photocatalytic action of the photocatalyst film 12, an active species diffusion distance control step for controlling a diffusion distance of the active species 40 in the processing solution 30 by a radical scavenger 42 added to the processing solution 30; and a processing step for chemically reacting the active species 40 with surface atoms 22 of the processing surface 20a and generating a chemical compound 50 to be eluted in the processing solution 30 to process the workpiece 20.