1. A document analysis apparatus comprising a computer with a central processing unit (CPU): the CPU being configured to function as a document collection acquisition unit which accepts an analysis object document to be an analysis object as a first document collection, and furthermore, accepts as an input a feature expression appearing during an attention period specified in advance in said first document collection, and for every said feature expression, acquires a collection of documents which have been issued, generated or updated during said attention period and in which said acquired feature expression has appeared, as a second document collection from among document collections including said first document collection; the CPU being configured to function as a context determination unit which, for every said feature expression, specifies a document corresponding to said analysis object document as a first feature expression containing document, among documents of said second document collection in which the feature expression has appeared, and furthermore, specifies a context which is common in two or more said first feature expression containing documents as the context of the feature expression, among contexts in which the feature expression has appeared in said first feature expression containing document; the CPU being configured to function as a context comparison determination unit which, for every said feature expression, specifies a document which does not correspond to said analysis object document as a second feature expression containing document, among documents of said second document collection in which the feature expression has appeared, and furthermore, performs comparison between a context in which the feature expression has appeared in said second feature expression containing document and a context which said CPU functioning as the context determination unit has specified; and the CPU being configured to function as a feature degree setting unit which, based on a result of comparison by said CPU functioning as the context comparison determination unit, gives a feature degree to said feature expression, or corrects a feature degree in the case where a feature degree has been given to said feature expression in advance, wherein said CPU functioning as the context determination unit, after specifying said first feature expression containing document, determines, for every said feature expression, whether a relation between the number of said first feature expression containing documents and the number of documents in which the feature expression has appeared within said second document collection fulfills a setting condition, and specifies said context in the case where said setting condition is not fulfilled, and wherein said CPU functioning as the context comparison determination unit performs a comparison between a context in which the feature expression has appeared in said second feature expression containing document and a context which said CPU functioning as the context determination unit has specified, with respect to each said feature expression for which said context has been specified.