1. A structured document transforming method for performing bidirectional transformation between a first structured document and a second structured document, wherein the first structured document and the second structured document each includes elements, comprising: preparing a transformation rule having primitive rules each of which is applicable to respective one of the elements specified in units of paths within the first structured document; performing a forward transformation which transforms the first structured document into the second structured document in accordance with the transformation rule; creating a transformation record wherein which primitive rules were applied and which paths were transformed in the forward transformation are recorded; editing the second structured document; and performing a backward transformation which transforms the second structured document into the first structured document while referring to the transformation record, the transformation rule, and the first structured document.