To set an alarm device at a mountainous region or the like at a low cost by providing a receiver or the like in a calculation and control circuit turning on and off and driving an alarm device display part, receiving a synchronous signal from the outside and correcting the turning-on and off phase of the display part.

The calculation and control circuit 11 of a road alarm device set along a road and for alarming and displaying the curve condition or the like of the road transmits a synchronous signal Sa to the outside every transmission cycle, and a synchronous signal Sb transmitted from the outside by the other alarm device is received by a receiver 11b. The turning-on and off phase of a display part 12a is corrected by the received signal, and, whenever the synchronous signal is received, the turning-on and off of the display part 12a is correctly synchronized with the turning-on and off of the display part of the other alarm device. The whole constitution is simplified and the alarm device can be set even at a region where a surface electric wave does not reach at a low cost. Thereby it can be easily set even at a mountainous region or the like.