1. A search device comprising: a character-string search index configured to register document data to be searched by dividing the document data into predetermined units; a relational-data expansion index configured to define a relationship between the respective document data and further define a type of the relationship; a character-string search unit configured to divide a search query into partial data in the predetermined units, acquire document data partially matching the partial data from the character-string search index, and calculate a similarity between the partial data and the respective document data; and a relational-data expansion search unit configured to acquire, from the relational-data expansion index, relational definition data of an expansion source document and an expansion destination document, and a relationship indicator corresponding to a type of relationship between the expansion source document and the expansion destination document, correct the similarity based on the relational definition data and a weighted factor associated with the relationship indicator, and arrange and output the expansion source document data in an order of the corrected similarity.