1. A structured document management system for managing a structured document, comprising: a decomposition part that decomposes an inputted structured document into plural partial structures in accordance with a setting and generating a relation between the partial structures as first structural information; a structural information registration part that generates a relation between elements in the partial structure for each of the partial structures decomposed by the decomposition part as second structural information so that a depth-first node order is assigned to each of the elements in the partial structure and a maximum node order of node orders of elements below the element is associated with the element; and an information retaining part that retains the first structural information generated by the decomposition part and the second structural information generated by the structural information registration part.