1. An information processing device, comprising: one or more non-transitory computer readable media storing: computer readable instructions; a content database configured to store contents with metadata and at least one keyword associated with each of the contents, wherein each metadata comprises data about the contents that is classified according to a plurality of items data and is defined and acquired at substantially a same time that the contents are acquired; a user database configured to store user identifiers and the data classified according to the plurality of items, each of the user identifiers being associated with a specific item from among the plurality of items, and each of the user identifiers being adapted to identify a user of each of the contents; and a processor configured to execute the computer readable instructions to provide: an acquiring unit configured to acquire new content and new metadata contemporaneous with and accompanying the new content; an identifier extracting unit configured to extract a user identifier from the new metadata acquired by the acquiring unit, the extracted user identifier identifying the user of the new content; a specific item specifying unit configured to specify the specific item associated with the user identifier that is stored in the user database and that corresponds to the extracted user identifier; a keyword extracting unit configured to extract keywords from the content database that are associated with existing metadata stored in the content database, the existing metadata including certain data that is identical to data included in the new metadata that is classified according to the specific item specified by the specific item specifying unit; 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 from which the user identifier is extracted by the identifier extracting unit.