Provided is a fine-processing agent which, when fine-processing a laminated film stacked at least with a silicon dioxide film and a silicon nitride film, can selectively fine-process the silicon dioxide film. Also provided is a fine-processing method utilizing the fine-processing agent. The fine-processing agent is characterized by including: (a) 0.01-15.0 weight % hydrogen fluoride and/or 0.1-40.0 weight % ammonium fluoride, (b) water, and (c) 0.001-10.00 weight % water-soluble polymer selected from among a group consisting of acrylic acid, ammonium acrylate, acrylic acid ester, acrylamide, styrenesulfonic acid, ammonium styrenesulfonate, and styrenesulfonic acid ester.