1. A transformation rule generation supporting method performed by a computer system for supporting 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, the method comprising: the computer system reading a graphical representation of the transformation rule from a storage device, the graphical representation of the transformation rule including at least one of: a plurality of links indicating a reduced transformation rule of a one-to-many transformation rule and 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; and a plurality of links indicating a reduced transformation rule of a many-to-one transformation rule and 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 in response to that the read graphical representation of the transformation rule is the one-to-many transformation rule, the computer system determining, according to an output sort order, an 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 in response to that the read graphical representation of the transformation rule is the many-to-one transformation rule, the computer system 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.