1. A machine translation system comprising: a storage unit configured to store a plurality of interpretation data items each including a first expression element representing a first expression of a first language, an interpretation word set element including the first expression, and a second expression element representing a second expression of a second language, wherein the first expression is to be translated into the second expression, and the first expression and the second expression are included in a category; an acquiring unit for acquiring: a predetermined expression of the first language, the predetermined expression being included in a structure of a to-be-translated document, wherein words or sentences are arranged regularly in the structure, and a first word set element including the predetermined expression and one or more expressions having equivalent positional relationship to the predetermined expression in the structure; a detecting unit for: detecting a subset from the plurality of interpretation data items, each of the subset including an expression element representing the predetermined expression and one or more word set elements, and performing matching between the first word set element and each of the one or more word set elements of the detected subset based on at least one of a number and an order of expressions of the first word set element; a selection unit for selecting, from the detected subset, an interpretation data item according to the matching; and a translation unit for translating the predetermined expression into the second language, using the selected interpretation data item.
6. The system according to claim 1 , wherein the first word set element contains data representing a chapter/section structure of the to-be-translated document, and a plurality of expressions in the first word set element are titles of chapters and titles of sections.