1. A processing system comprising: a microprocessor coupled to a memory, wherein the microprocessor is programmed to perform a time-series analysis of keywords, by implementing: a clustering unit segmenting by performing at least one of clustering or 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 a time-series analysis unit acquiring a frequency distribution showing variation of the frequencies of occurrence of the pieces of document data by performing 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.