1. A search device comprising: a character-string search index in which document data to be searched and partial data by dividing the document data into predetermined units are registered; a relational-data expansion index in which a relationship between the respective document data and a type of the relationship are defined; a character-string search unit that divides a search query into partial data in the predetermined units, and that acquires the document data partially matching the partial data from the character-string search index to calculate a similarity thereof; and a relational-data expansion search unit that acquires document data of an expansion destination defined to have the relationship with respect to the expansion source document data from the relational-data expansion index using the document data acquired by the character-string search unit as an expansion source, and that corrects the similarity of the document data of the expansion source by using a weight according to the type of the relationship and the similarity of the expansion destination document data, and that arranges and outputs the expansion source document data in an order of the corrected similarity.