1. An information processing device, comprising: a content database configured to store contents with metadata and at least one keyword associated with each of the contents, wherein the metadata is data accompanying each of the contents and includes a plurality of items of data; a user database configured to store user identifiers with a specific item of the metadata associated with each of the user identifiers, the user identifiers each of which is an identifier adapted to identify a user of each of the contents; an acquiring unit configured to acquire new content and new metadata accompanying the new content; an identifier extracting unit configured to extract a user identifier for identifying a user of the new content, from the new metadata acquired by the acquiring unit; a specific item specifying unit configured to specify a specific item associated with the extracted user identifier, based on the user database; a keyword extracting unit configured to extract keywords associated with data identical to data of the specified specific item included in the new metadata, from the content database; a keyword selecting unit configured to select at least one keyword from the keywords extracted by the keyword extracting unit; and a registration unit configured to register, in the content database, the at least one keyword selected through the keyword selecting unit, in association with the new content and the new metadata.