PURPOSE: To warn danger and to obtain a strong appealing force by dynamically changing a display color in correspondence to the condition of an over signal level.

CONSTITUTION: The titled device equips a red light source part 5 in which an outgoing light flux is controlled by the output power of a red system control part 1 and a blue-green system light source part 15 in which the outgoing light flux is controlled by the output power of a blue-green system control part 11. A mixed color display board is projected so that a red system outgoing light flux 5A of the light source part 5 and a green (blue) system outgoing light flux 15A of the light source part 15 can be overlapped to the same area of the mixed color display board 9. At a time point an input signal achieves a limiting level E2, the minimum light flux is outgoing from a light source 16 and the maximum light flux is outgoing from a light source 6. Then, according to the ratio of the outgoing light fluxes of the both light sources, the display 9 displays the mixed color whose color tone is successively changed, for example, from yellow-green through yellow to orange. After that, the dangerous display in red is executed. Thus, the warning function and the strong appealing force can be obtained.