Abstract: A micromachining processing agent and a micromachining processing method capable of selectively micromachining a silicon oxide film when a laminated film including at least a silicon nitride film, a silicon oxide film, and a silicon alloy film is micromachined. The micromachining processing agent is used for micromachining of a laminated film including at least a silicon oxide film, a silicon nitride film, and a silicon alloy film. The micromachining processing agent contains: (a) 0.01 to 50 mass % of hydrogen fluoride; (b) 0.1 to 40 mass % of ammonium fluoride; (c) 0.001 to 10 mass % of a water-soluble polymer; (d) 0.001 to 1 mass % of an organic compound having a carboxyl group; and (e) water as an optional component, in which the water-soluble polymer is at least one selected from a group consisting of acrylic acid, ammonium acrylate, acrylamide, styrenesulfonic acid, ammonium styrenesulfonate, and styrenesulfonic acid ester.