10. The translation apparatus as set forth in claim 1 , wherein denoting the new source language translation unit as D i and the compared source language translation unit stored in the translation memory as S k , the comparator computes a similarity parameter δ between D i and S k given by: 
 δ= f (δ T ,δ I ,δ O ) where δ T indicates a textual similarity between D i and S k , δ I indicates a similarity between an inner structure extending from the identified structural position and corresponding structural information of S k , δ O indicates a similarity between an outer structure connecting with D i and corresponding structural information of S k , and f denotes a function that combines δ T , δ I , and δ O .
12. The translation apparatus as set forth in claim 10 , wherein the structural information for at least some of the source language translation units stored in the translation memory indicates that the document from which the source language translation unit was derived is an unstructured document.