6. A machine-translation apparatus comprising: an example storage unit that stores target language examples in a target language and source language examples in a source language in correspondence with one another, each of the target language examples having a meaning equivalent to a meaning of a corresponding one of the source language examples; an input receiving unit that receives an input sentence in the source language; a searching unit that conducts a search in the example storage unit for one of the target language examples corresponding to one of the source language examples that either matches or is similar to the input sentence; a first translating unit that generates a first reverse-translated sentence by translating the one of the target language examples found in the search into the source language; a second translating unit that generates, using a processor, a target language sentence by translating the input sentence into the target language, and generates a second reverse-translated sentence by translating the generated target language sentence into the source language; a detecting unit that detects a difference portion between the first reverse-translated sentence and the second reverse-translated sentence; a generating unit that generates an output sentence by adding the difference portions to the input sentence an output unit that outputs, to a user, the output sentence to which the difference portions are added; and a selection receiving unit that receives a selection of one of the output difference portions, wherein: the translating unit further generates the target language sentence by translating a combination of the input sentence and the selected one of the difference portions, and the output further outputs the target language sentence.
6. A machine-translation apparatus comprising: an example storage unit that stores target language examples in a target language and source language examples in a source language in correspondence with one another, each of the target language examples having a meaning equivalent to a meaning of a corresponding one of the source language examples; an input receiving unit that receives an input sentence in the source language; a searching unit that conducts a search in the example storage unit for one of the target language examples corresponding to one of the source language examples that either matches or is similar to the input sentence; a first translating unit that generates a first reverse-translated sentence by translating the one of the target language examples found in the search into the source language; a second translating unit that generates, using a processor, a target language sentence by translating the input sentence into the target language, and generates a second reverse-translated sentence by translating the generated target language sentence into the source language; a detecting unit that detects a difference portion between the first reverse-translated sentence and the second reverse-translated sentence; a generating unit that generates an output sentence by adding the difference portions to the input sentence an output unit that outputs, to a user, the output sentence to which the difference portions are added; and a selection receiving unit that receives a selection of one of the output difference portions, wherein: the translating unit further generates the target language sentence by translating a combination of the input sentence and the selected one of the difference portions, and the output further outputs the target language sentence. 8. The apparatus according to claim 6 , wherein the example storage unit further stores priority levels in correspondence with the target language examples, the priority levels indicating degrees with which the target language examples are prioritized, respectively, the searching unit conducts a search by giving a higher priority to some of the target language examples having higher priority levels than other target language examples having lower priority levels, and the apparatus further comprises an updating unit that raises, by a predetermined value, the priority level of the target language example from which the reverse-translated sentence containing the selected difference portion are generated.