PURPOSE: To realize a curved road alarm device giving an alarm to dangerous car speed by finding a radius of curvature of a curved road without finding the radius of curvature of the curved road from a white line coordinate in a two-dimensional picture image and without setting a transmitter on the curved road.

CONSTITUTION: A radius of curvature of a curved road is decided by collating a letter pattern decided by applying binarization treatment and edge treatment to a picture image signal obtained from a two-dimensional camera picking up an image of a curve curvature display sign set on a curved road and a letter pattern of a previously memorized curve curvature, previously set allowed car speed is found from this radius of curvature, and when detected self car speed 2 is higher than this allowed car speed, it is judged as over speed and an alarm is energized for a specified period of time.