1. A computer system for supporting a document data search based on a keyword, comprising: an extraction unit that extracts a plurality of keywords from search target document data; a graph generation unit that generates a graph including a plurality of objects respectively representing the plurality of keywords extracted by the extraction unit and being classified into a plurality of clusters; a search condition statement generation unit that generates a search condition statement by use of a keyword, in accordance with a user operation to select a specific cluster among the plurality of clusters in the graph generated by the graph generation unit, the keyword being represented by an object that belongs to the specific cluster; and among keywords that appear in the search target document data, the extraction unit extracts keywords that appear at a specific level of frequency, as the plurality of keywords corresponding to the specific level, and in accordance with a user operation to specify the specific level, the graph generation unit generates the graph including a plurality of objects respectively representing the plurality of keywords corresponding to the specific level and being classified into a plurality of clusters.