An ion milling device capable of high-speed milling is realized even for a specimen containing a material having an imide bond. Therefore, the ion milling device includes: a vacuum chamber 6 configured to hold a specimen 3 in a vacuum atmosphere; an ion gun 1 configured to irradiate the specimen with a non-focused ion beam 2; a vaporization container 17 configured to store a mixed solution 13 of a water-soluble ionic liquid and water; and nozzles 11, 12 configured to supply water vapor obtained by vaporizing the mixed solution to a vicinity of a surface of the specimen processed by the ion beam.