Provided is a steering assistance device, and a demarcation line detection unit 100 detects left and right lane demarcation lines for a lane in which a vehicle is travels. A horizontal velocity measurement unit 101 calculates the horizontal velocity, which is a velocity component of the vehicle in a direction perpendicular to the lane. An assistance unit 11 performs steering assistance for the vehicle so that the vehicle travels parallel to the left and right lane demarcation lines. In a case where the horizontal velocity equal to or greater than a prescribed velocity and the center of the vehicle is separated from the center between the left and right lane demarcation lines by equal to or greater than a prescribed distance, a steering control unit 102 starts the steering assistance of the vehicle by the assistance unit 11.