1. A translation system comprising: a storage device; a processor; a bilingual data storage section, a plurality of pieces of first language simple sentence data corresponding to a plurality of first language simple sentences in a first language and a plurality of pieces of second language simple sentence data corresponding to a plurality of second language simple sentences in a second language being stored in the bilingual data storage section while being associated with each other so that the first language simple sentences and the second language simple sentences respectively make pairs; and a target language simple sentence data output section which outputs target language simple sentence data corresponding to a target language simple sentence which is a translation of a given source language simple sentence based on source language simple sentence data corresponding to the source language simple sentence, the target language simple sentence data output section receiving first-language-source-language simple sentence data corresponding to a first-language-source-language simple sentence in the first language, and selecting first language simple sentence data from the plurality of pieces of the first language simple sentence data stored in the bilingual data storage section based on the received first-language-source-language simple sentence data; and the target language simple sentence data output section outputting the second language simple sentence data associated with the selected first language simple sentence data as the target language simple sentence data; wherein the first language simple sentence data is stored in the bilingual data storage section while being classified into a plurality of groups; wherein one piece of the first language simple sentence data classified into each of the groups is designated as representative data; and wherein the target language simple sentence data output section selects one piece of the first language simple sentence data designated as the representative data.