1. A keyword outputting apparatus comprising: a document input device configured to input a plurality of documents having date information in a designated period; a keyword analysis device configured to obtain a plurality of word vectors represented by the documents by analyzing a plurality of keywords contained in each of the documents; a topic cluster extraction device configured to extract a pair of documents as a topic cluster from said documents, the pair of documents having a maximum inner product of the corresponding word vectors and belonging to an identical topic; a keyword extraction device configured to extract a predetermined number of keywords as a characteristic keyword group from the topic cluster in descending order of appearance frequency; a topic structurization determination device configured to determine whether the topic is configured to be structurized, by segmenting the topic cluster into subtopic clusters with reference to one of a number of the documents, a variance of dates contained in the documents, and a C-value of keyword in the documents, as a determination criterion; a control device configured to control the topic cluster extraction device to repeatedly extract a subtopic cluster until it is not determined that a subtopic of the subtopic cluster is not configured to be structurized, and to control the keyword extraction device to extract another characteristic keyword group from the extracted subtopic cluster; and a keyword presentation device configured to present the characteristic keyword group in the subtopic cluster upon arranging the keyword group on the basis of the date information.