1. A document reading apparatus presenting a plurality of documents designated as reading documents by a user, comprising: information processor having a memory, including a thematic hierarchy recognizing device recognizing a thematic hierarchy of each of the plurality of documents: by repeating a process for detecting a set of thematic boundaries in each layer of the thematic hierarchy, with each of a plurality of different window widths, wherein each of the thematic boundaries is detected based on a lexical cohesion score obtained from a similarity of vocabularies that appear in two adjacent windows with each of the window widths at each location in each of the plurality of documents, and by correlating first and second thematic boundaries locating closely and detected with smaller and larger window widths, respectively; a topic extracting device extracting a topic that commonly appears in the plurality of documents based on the thematic hierarchies recognized; and a topic relation presenting device taking out a description part corresponding to the topic extracted from each of the plurality of documents and outputting the description parts taken out as related passages among said plurality of documents, wherein regarding a topic set that comprises topics of various grading in the thematic hierarchies recognized, the topic extracting device calculates a relevance score between topics of the topic set based on lexical similarity of description parts corresponding to each topic of the topic set, and extracts a topic set having a relevance score equal to or more than a threshold that is set based on inclusive relationship of topics, and the threshold, corresponding to any topic among topics which constitutes a target extraction of topic set, is a maximum value of calculated relevance score related to a topic which is included in a subtree in thematic hierarchies.