2. The machine translation method according to claim 1 , wherein the method further comprising: changing an output form of the forward-translated sentence corresponding to the selected backward-translated sentence in accordance with which of a voice information via the microphone and a text information via the text input device is received as the translation-source sentence.
2. The machine translation method according to claim 1 , wherein the method further comprising: changing an output form of the forward-translated sentence corresponding to the selected backward-translated sentence in accordance with which of a voice information via the microphone and a text information via the text input device is received as the translation-source sentence. 3. The machine translation method according to claim 2 , wherein the information output device has a speaker, a display, and a processor; and wherein the machine translation method further comprises: outputting, when the translation-source sentence is received via the microphone, the forward-translated sentence corresponding to the selected backward-translated sentence via the speaker; and outputting, when the translation-source sentence is received via the input device, the forward-translated sentence corresponding to the selected backward-translated sentence via the display.