According to the present invention, a desired shape is etched by means of highly-precise control and long-term stabilization of the thickness of a film that is deposited on a pattern during cyclical etching that proceeds by the repetition of a deposition step and an etching step. The present invention comprises: a deposition step (S1) for introducing a reactive, depositing gas into a treatment chamber to form a deposit layer on the surface of an etching pattern of an etching substrate; an etching step (S2) for removing the product of a reaction between the deposit layer and the surface of theetching pattern; and a step (S3) for shining light onto the etching pattern during the deposition steps that form the deposit layer during cyclical etching in which the deposition step and the etchingstep are alternatingly executed to produce a fine pattern and monitoring the change in the film thickness of the deposit layer on the basis of the change in interference light of a specific wavelength that has been reflected by the etching pattern. Treatment conditions for the steps that will form the deposit layer during subsequent cycles of the cyclical etching are determined such that a deposited film thickness index that is calculated from the monitored change in the film thickness of the deposit layer falls within a prescribed range in comparison with reference data.