1. A retrieval device comprising: a processor that performs semantic analysis on a query and generates one or more semantic structures where each structure is expressed by a graph; generates retrieval keys corresponding to combinations of nodes connected directly or indirectly in the semantic structures, in addition to retrieval keys corresponding to minimum units of semantic connections between nodes in the generated semantic structures; and retrieves relevant documents whose sentences are matched to combinations of nodes, by using the generated retrieval keys, in the semantic structures stored in an index for retrieval on a database storing the documents.