PROBLEM TO BE SOLVED: To discriminate a part set as an operation object of operation input at a specific time to a document.SOLUTION: A frequent operation determining means 5 selects a plurality of defined operation information whose input time is continuous in time series, and extracts, as continuous deletion character strings, a group of deleted character strings of difference information associated with the plurality of selected defined operation information respectively. The frequent operation determining means 5 determines the continuous deletion character strings whose extracted frequency is a predetermined number or more, as frequent continuous deletion character strings. An estimating means 6 generates a group of deletion character string included in the difference information and deletion character strings included in the difference information associated with the defined operation information whose input time to the same correction document as the undefined operation information is continuous in time series. The estimating means 6 detects difference information in which the group of deletion character strings generated using the difference information coincides with the frequent continuous deletion character strings, out of the difference information associated with the undefined operation information, and estimates a position indicated in the detected difference information, as a position operated when recording the undefined operation information.
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