1. A transformation rule generation supporting apparatus configured to support generation of a transformation rule for transforming a transformation-source structured document having a hierarchical structure based on physical disposition of data in the document into a transformation-target structured document having a hierarchical structure based on a logical structure of data content, comprising: a user interface configured to generate, display, and edit graphical representation of the transformation rule, the user interface allowing generation of a reduced rule of at least one of a one-to-many transformation rule and a many-to-one transformation rule, the one-to-many transformation rule being indicated by a plurality of links mapping one node representing an input element that is an element in the transformation-source structured document to a plurality of nodes each representing an output element that is an element in the transformation-target structured document, the many-to-one transformation rule being indicated by a plurality of links mapping a plurality of nodes each representing an input element in the transformation-source structured document to one node representing an output element in the transformation-target structured document; and a restoration unit configured to determine a transformation rule in text form from the graphical representation, wherein, for the one-to-many transformation rule, the restoration unit restores the reduced rule by determining, according to an output sort order, the output order in which each of the output elements represented by the plurality of nodes is output for the input element represented by the one node, the output sort order being a depth-first order in the hierarchical structure of the transformation-target structured document, and, for the many-to-one transformation rule, the restoration unit restores the reduced rule by determining an output target to which the output element represented by the one node is output for each of the input elements represented by the plurality of nodes with reference to an output target of an input element located immediately before each of the input elements in a list in an input sort order of one or more input elements mapped to an output element that is a parent of the output element, the input sort order being a depth-first order in the hierarchical structure of the transformation-source structured document.