A lane departure warning system comprising: an image capturing device; a compartment line detection unit that is configured to detect a compartment line (31, 32) in an image of a running lane on which a vehicle is traveling, the image being captured by the image capturing device; a dirtiness detection unit that is configured to detect a degree of dirtiness on the lens of the image capturing device when the image capturing device is arranged outside the vehicle interior or on the partition between an outside of a vehicle interior of the vehicle and the image capturing device when the image capturing device is arranged in the vehicle interior, wherein the degree of dirtiness corresponds to a ratio of the area of the dirtiness region to the total area of the image; a departure judgment unit that is configured to judge whether or not the vehicle departs from the compartment line (31, 32) when the distance from the vehicle to the compartment line (31, 32) becomes smaller than a predetermined threshold value; and a warning judgment unit that when it is judged by the departure judgment unit that the vehicle departs from the compartment line (31, 32), is configured to output a warning signal, wherein the compartment line detection unit is configured to calculate the distance from the vehicle to the compartment line as a weighted sum of the current and previous distance with p being the weight for the current distance, 1-p being the weight for the previous distance, and p being within the range 0≤p≤1, the weight becoming smaller as the degree of the dirtiness increases; and the departure judgment unit is configured to decrease the threshold value with increasing degree of the dirtiness. an image capturing device; a compartment line detection unit that is configured to detect a compartment line (31, 32) in an image of a running lane on which a vehicle is traveling, the image being captured by the image capturing device; a dirtiness detection unit that is configured to detect a degree of dirtiness on the lens of the image capturing device when the image capturing device is arranged outside the vehicle interior or on the partition between an outside of a vehicle interior of the vehicle and the image capturing device when the image capturing device is arranged in the vehicle interior, wherein the degree of dirtiness corresponds to a ratio of the area of the dirtiness region to the total area of the image; a departure judgment unit that is configured to judge whether or not the vehicle departs from the compartment line (31, 32) when the distance from the vehicle to the compartment line (31, 32) becomes smaller than a predetermined threshold value; and a warning judgment unit that when it is judged by the departure judgment unit that the vehicle departs from the compartment line (31, 32), is configured to output a warning signal, wherein the compartment line detection unit is configured to calculate the distance from the vehicle to the compartment line as a weighted sum of the current and previous distance with p being the weight for the current distance, 1-p being the weight for the previous distance, and p being within the range 0≤p≤1, the weight becoming smaller as the degree of the dirtiness increases; and the departure judgment unit is configured to decrease the threshold value with increasing degree of the dirtiness.