A road information detector acquires road information by emitting a beam via a variable directivity transmission antenna to a road sign having plural flat portions and includes a transceiver that controls the antenna to scan the beam by switching an emission angle including an azimuth angle relative to the front direction of a vehicle and receives, as reflected wave signals, waves reflected by the flat portions, a distance and reflected wave intensity detector that detects a distance between the vehicle and each of the flat portions and a reflected wave intensity of each of the reflected waves, an emission angle detector that detects the emission angle based on the reflected wave signal, and a road information analyzer that acquires the road information by generating a heat map based on the emission angles, distances, and reflected wave intensities and analyzing the heat map using a threshold value of the wave intensity.