1. An apparatus for providing assistance in detecting mistranslation in a translated document obtained by performing machine translation of an original document, the apparatus comprising: a processor; a memory containing instructions that are executable by the processor; a first acquisition unit operable to obtain a word included in the translated document; a second acquisition unit operable to obtain a first value representing the search results of performing a web search of a first document data group and a second value representing the search results of performing a web search of a second document data group including the first document data group for pieces of document data related to the word obtained by the first acquisition unit, wherein the first document data group is obtained from a specific website known to be relevant in a subject matter field for the obtained word, and wherein the second document data group is obtained through a general web search; a generation unit operable to generate, based on a comparison of the ratio of the first value and the second value with a predetermined threshold value, an index indicating adequacy of the word obtained by the first acquisition unit as a translated word in a subject matter field corresponding to the first document data group; and an output unit operable to output the index generated by the generation unit, wherein the processor is operable to execute instructions stored in the memory to coordinate the actions performed by the first acquisition unit, the second acquisition unit, the generation unit and the output unit.