In a communication system in which translation is achieved in a bidirectional manner between a first language terminal and a second language terminal, words appearing in sentences inputted from the first-language terminal are memorized so that the memorized words are preferentially selected when determining equivalents in later translations of sentences inputted from the second-language terminal, and vice versa. As means for memorizing an occurring word, a sentence number assigned to an input sentence containing the word may be stored in a word dictionary as activation degree information of the word or an occurring word file may be disposed for sequentially storing therein the occurring word. These items may be stored in combination with terminal user IDs. Various types of function allocation methods are possible. For instance, an input sentence can be analyzed by an input terminal so as to send the analyzed information to a destination terminal, which then generates a translated sentence from the received information a translated sentence obtained by a terminal of an input side can be sent to a terminal of a destination side.