A thin specimen producing method acquires a work amount in a 1-line scan by an FIB under a predetermined condition, measures a remaining work width of a thin film on an upper surface of a specimen by a microscopic length-measuring function, determines a required number of scan lines of work to reach a predetermined width by calculation, and executes a work to obtain a set thickness. The work amount in a one-line scan by the FIB under the predetermined condition is determined by working the specimen in scans of plural lines, measuring the etched dimension by the microscopic length-measuring function, and calculating an average work amount per one-line scan.