The present invention proposes an ECU and a lane departure warning system capable of improving safety of all traveling vehicles including a host vehicle by using an environment sensor to warn another vehicle about departure of the other vehicle from a lane.
An ECU (13) detects the departure from the lane. The ECU (13) determines whether another vehicle (100) leaves the lane on the basis of a detection signal from an environment sensor (11), and visually or aurally warns the other vehicle (100) in the case where the other vehicle (100) leaves the lane.