PURPOSE: To increase the kind of an instruction and data which can be inputted without increasing the number of input keys by providing an external input/ output device provided with a means for selecting a mutual instruction by the same inversion key.

CONSTITUTION: In an external input/output device, a controller 1 is provided and to the controller 1, an input device 2, a display device 5 and a RAM 6 are connected and also, an external operation data recorder 8 is connected so as to be freely attachable/detachable. In the input device 2, a key input item (occupied taxi-vacant taxi, general road-expressway) input part 3 is provided and the specific key of the key input item input part 3 is constituted of an inversion key. By a key input, whether an inputted key is an inversion key or not is decided and in the case it is not the inversion key, the state of an instruction corresponding to the input key is displayed on the display device 5 and control is finished. In the case it is the inversion key, the state of an instruction before its key is changed, displayed on the display device 5 and control is finished.