1. A method for processing a time-series analysis of keywords, the method comprising: segmenting, with a processor, by performing at least one of clustering and classifying, pieces of document data based at least in part on frequencies of occurrence of keywords in the pieces of document data, wherein the pieces of document data include a description in a natural language, the segmenting resulting in creating at least one document cluster and at least one keyword cluster; and acquiring a frequency distribution showing variation of the frequencies of occurrence of the pieces of document data by performing, with the processor, at least one of: a time-series analysis of frequencies of occurrence of pieces of document data containing individual keywords in at least one document cluster, and a time-series analysis of frequencies of occurrence of pieces of document data containing at least one keyword cluster.