1. A data input device that includes two inputs including a character string input that accepts a character string and a voice input that accepts voice input, a display that displays the character string, and a speech recognition dictionary, the data input device comprising: a dynamic speech recognition dictionary generator which, in operation, extracts phrases of which heads match a head of the character string inputted through the character string input from phrases stored in the speech recognition dictionary, and generates a dynamic speech recognition dictionary that stores difference phrases, the difference phrases each being part of an extracted phrase excluding a common phrase that is common among the extracted phrases; a display controller which, in operation, displays the difference phrases of an input character string candidate, the difference phrases being highlighted on the display and excluding the common phrase; a speech recognizer which, in operation, carries out speech recognition of voice inputted through the voice input by using the dynamic speech recognition dictionary; an input character string confirmer which, in operation, confirms the input character string candidate that includes a difference phrase recognized by the speech recognizer as an input character string; and an input switcher which, in operation, switches between the character string input and the voice input based on a running state of a vehicle, wherein, in a case of receiving a changed state of the vehicle from stopping to starting before a whole character string is finished inputting, the input switcher switches from the character string input to the voice input, the display controller displays the difference phrases of the input character string candidate with the common phrase that has already been confirmed by the character string input, the difference phrases being highlighted on the display, and the input character string confirmer confirms the input character string candidate using the highlighted difference phrase recognized by the speech recognizer as the input character string, and at least one of the dynamic speech recognition dictionary generator, the display controller, the speech recognizer and the input character string confirmer is included in a processor.