To accomplish use as a portable alarm signal display device in the night by switching a mode of a display device to an emergency signal mode, which is suitable for an end user to inform a peripheral traffic condition in the night, by means of a built-in battery storing energy when the display device is detached from an automobile.

When a display device 20 is detached from an automobile, a power source is switched from an automobile power source to built-in batteries 24, 25, 26, and electric power is supplied to the display device 20. A prefocus type light emitting diode 37 is arranged in a proper position in a PCB assembly 23 of a display module 38 arranged in the display device 20, and it is automatically turned on only in the night by means of an optical sensor 35. In this process, intensity of light emitted from the light emitting diode 37 is increased, and a switch to an emergency signal mode is carried out in the display device 20. In this way, the display device 20 can be used as a portable alarm signal display device in the night.