To provide an on-vehicle display device of which not only cost reduction but weight reduction and miniaturization are attained by simple structure, which is easily mounted on and detached from a roof of a vehicle and can be used even in a state in which it is detached down from the vehicle.

A supporting mechanism 11 is attachably/detachably mounted on a carrier B previously provided on the roof A1 of the vehicle A, a display main body 30 is linked on the supporting mechanism 11 to be erectable and collapsible. The display main body 30 is held so that it can be selectively and manually switched among a front display state that a front panel 31 is approximately vertically erected forward, a rear display state that the front panel 31 is approximately vertically erected backward and a stored state that the front panel 31 approximately horizontally falls down when the display main body 30 is linked with the supporting mechanism 11.