A retrieval device, from semi-structured data formed by item elements and value elements, creates aggregated structure information from the semi-structured data by aggregating the plurality of child item elements having the same name and directly connected to parent item elements having the same name, into one child item element. The device receives a retrieval request with an output condition and a constraint condition specified on the aggregate structure information. The device determines whether or not a plurality of query are created according to the retrieval request by checking a branch point at the common item elements between the output condition and the constraint condition in the aggregated structured information. And the device outputs a re-aggregate structure information corresponded to the query showing the branch point which is not expressed in the aggregate structure information to the display device.