1. An information processing device comprising: a memory controller to store in a memory: a plurality of character strings included in a voice text obtained by performing a voice recognition on voice data; a node index indicating start position information and end position information of each of the character strings in the voice text; a voice recognition score obtained by performing the voice recognition; and a voice index of voice position information indicating a position of each of the character strings in the voice data; a detector to detect reproduction section information which indicates a reproduced section of the voice data; an obtainer to obtain reading information, which represents at least a part of a character string representing reading of a phrase in a text written down from the voice data that has been reproduced, and obtain insertion position information, which indicates a character insertion position in the written text; a searcher to specify, as a target character string for searching, a character string that, from among the plurality of character strings stored in the memory, has the corresponding voice position information included in the reproduction section information, and search the specified character string for a character string that includes reading indicated by the reading information; a determiner to, when a value of the voice recognition score corresponding to the character string obtained by the searcher is equal to or greater than a display threshold value, determine to display the character string obtained by the searcher; a display controller to display the character string, which is determined by the determiner, on a display unit; a history updater to perform storage control to store, in a candidate history memory, candidate history data in which the character string obtained by the searcher, the voice recognition score, and the character insertion position are associated, and to update the candidate history data according to a change in a text; a selector to, when the character string displayed by the display controller is subjected to a selection operation, select the character string; and a threshold value updater to decide on the display threshold value, which is used in comparison with a voice recognition score by the determiner, using at least one of a voice recognition score of the candidate history data and a voice recognition score of the character string selected by the selector.