A solid object detection device for detecting a solid object traveling in an adjacent traffic lane that is adjacent across a dividing line serving as a boundary to the traffic lane traveled by a vehicle, the solid object detection device comprising: image-capturing means adapted for capturing an image including the dividing line and a predetermined region (A1, A2) of the adjacent traffic lane, the image-capturing means being installed in the vehicle; solid object assessment means adapted for assessing whether or not a solid object is present in the predetermined region image captured by the image-capturing means; lateral position detection means adapted for detecting, from the image captured by the image-capturing means, the vehicle-widthwise distance (Δy) between the dividing line and the vehicle position in the lane traveled by the vehicle, traffic lane change detection means adapted for detecting a traffic lane change made by the vehicle, characterized by: region setting means adapted for enlarging the size of the predetermined region, positioned on the side where the dividing line is located, farther outward in the vehicle-width direction correspondingly with respect to the increase in vehicle-widthwise distance to the dividing line detected by the lateral position detection means, such that the greater is the detected vehicle-widthwise distance to the dividing line, the farther outward in the vehicle-width direction the region setting means enlarges the size of the predetermined region; and the amount by which the size of the predetermined region is enlarged outward in the vehicle-width direction being reduced by the region setting means, for a certain time duration, when a traffic lane change made by the vehicle is detected by the traffic lane change detection means. image-capturing means adapted for capturing an image including the dividing line and a predetermined region (A1, A2) of the adjacent traffic lane, the image-capturing means being installed in the vehicle; solid object assessment means adapted for assessing whether or not a solid object is present in the predetermined region image captured by the image-capturing means; lateral position detection means adapted for detecting, from the image captured by the image-capturing means, the vehicle-widthwise distance (Δy) between the dividing line and the vehicle position in the lane traveled by the vehicle, traffic lane change detection means adapted for detecting a traffic lane change made by the vehicle, characterized by: region setting means adapted for enlarging the size of the predetermined region, positioned on the side where the dividing line is located, farther outward in the vehicle-width direction correspondingly with respect to the increase in vehicle-widthwise distance to the dividing line detected by the lateral position detection means, such that the greater is the detected vehicle-widthwise distance to the dividing line, the farther outward in the vehicle-width direction the region setting means enlarges the size of the predetermined region; and the amount by which the size of the predetermined region is enlarged outward in the vehicle-width direction being reduced by the region setting means, for a certain time duration, when a traffic lane change made by the vehicle is detected by the traffic lane change detection means. region setting means adapted for enlarging the size of the predetermined region, positioned on the side where the dividing line is located, farther outward in the vehicle-width direction correspondingly with respect to the increase in vehicle-widthwise distance to the dividing line detected by the lateral position detection means, such that the greater is the detected vehicle-widthwise distance to the dividing line, the farther outward in the vehicle-width direction the region setting means enlarges the size of the predetermined region; and the amount by which the size of the predetermined region is enlarged outward in the vehicle-width direction being reduced by the region setting means, for a certain time duration, when a traffic lane change made by the vehicle is detected by the traffic lane change detection means.