Micromachining processing agent and micromachining processing method

Provided are: a micromachining processing agent that makes it possible to selectively perform micromachining of a silicon oxide film when performing micromachining of a layered film including at least a silicon nitride film, a silicon oxide film, and a silicon alloy film; and a micromachining processing method. This micromachining processing agent is for use in micromachining of a layered film including at least a silicon oxide film, a silicon nitride film, and a silicon alloy film. The micromachining processing agent includes (a) 0.01-50 mass% of hydrogen fluoride, (b) 0.1-40 mass% of ammonium fluoride, (c) 0.001-10 mass% of a water-soluble polymer, (d) 0.001-1 mass% of an organic compound having a carboxyl group, and (e) water as an optional component. The water-soluble polymer selectively performs micromachining of the silicon oxide film and is at least one substance selected from the group consisting of acrylic acid, ammonium acrylate, acrylamide, styrenesulfonic acid, ammonium styrene sulfonate, and an e