6. The method according to claim 1 , further comprising: receiving a search query, narrowing down a set of pieces of document data by the search query to create a narrowed set of pieces of document data; acquiring, from a time series of a keyword in the narrowed set of pieces of document data a time series of each keyword cluster in the narrowed set of pieces of document data; and performing a frequency analysis of the time series of the keyword cluster.
6. The method according to claim 1 , further comprising: receiving a search query, narrowing down a set of pieces of document data by the search query to create a narrowed set of pieces of document data; acquiring, from a time series of a keyword in the narrowed set of pieces of document data a time series of each keyword cluster in the narrowed set of pieces of document data; and performing a frequency analysis of the time series of the keyword cluster. 12. The method according to claim 6 , wherein acquiring a time series of each keyword cluster includes performing a time-series analysis of a small number of keyword clusters according to an equation: c _ ⁡ ( t ) = ∑ x ⁢ s c ⁡ ( x ) ⁢ x _ ⁡ ( t ) where c (t) is a sum regarding all keywords belonging to a cluster in a time series, s c (x) is a weight of a keyword x in a cluster c, and x (t) is a normalized time series of a frequency of occurrence of pieces of document data containing each keyword, the time series having been smoothed.