An onboard display device includes multiple screen data that define plural types of screen designs including a character string display frame with equal size; plural types of font data; and multiple character string tables which record character strings to be displayed on a screen with one of the screen designs defined by the plural types of screen data, are each provided for one of the plural types of font data, and include character strings whose written forms are altered according to character size defined by the font data so that the character strings can be contained in the display frame, wherein the decision unit 12 decides a display type to be used for screen display on a display unit, and the display unit 6 displays the screen using the screen data, font data and character string corresponding to the display type decided by the decision unit 12.