1. A content searching device which searches content from a content database with a use of a related keyword which is related to a keyword describing content, the content database storing the content for each content attribute showing a classification of the content, and said content searching device comprising: a dictionary database which stores a degree of relevance, among a plurality of keywords, for each related segment indicating a time segment, the plurality of keywords each describing the content which (i) belongs to the classification indicated by the content attribute, and (ii) is included in the related segment; a related segment calculating unit configured to calculate, for each content attribute, a related segment which is defined in order for first content and second content to be included in a same time segment, the related segment being calculated based on whether or not a degree of difference, for each content attribute, calculated out of a plurality of first keywords and a plurality of second keywords meets a predetermined reference value, and the plurality of the first keywords each describing the first content to be stored in said database, and the plurality of the second keywords each describing the second content having been stored in said content database; a dictionary updating unit configured to update the degree of relevance, stored in said dictionary database, regarding the content included in the related segment calculated by said related segment calculating unit, the degree of relevance being updated with a use of the related segment, and the degree of relevance, among the plurality of keywords, being calculated for each content attribute; and an output generating unit configured to generate output information used for outputting, for each related segment, the related keyword relating to a keyword entered by a user, the output information being generated based on the degree of relevance stored in said dictionary database, wherein said related segment calculating unit is configured to calculate (i) a related segment defined by a time segment corresponding to the first content in the case where the degree of difference exceeds the predetermined reference value, and (ii) a related segment defined by a time segment which is a sum of a latest related segment stored in said dictionary database and the time segment corresponding to the first content in the case where the degree of difference is equal to the predetermined reference value or below.