Lane departure warning system

A lane departure warning device includes: a compartment line detection unit that detects a compartment line 31, 32 in an image of a running lane 30 on which a vehicle is traveling, the image being captured by an image capturing device 101 which the vehicle has; a dirtiness detection unit 2 that detects dirtiness on any one of a lens of the image capturing device 101 or a partition between an outside of a vehicle interior of the vehicle and the image capturing device 101; a departure judgment unit 3 that judges whether or not the vehicle departs from the compartment line 31, 32; and a warning judgment unit 5 that when it is judged that the vehicle departs from the compartment line 31, 32, outputs a warning signal based on a degree of the dirtiness detected. A position of the vehicle with respect to the compartment line 31, 32 when the warning signal is output varies depending on the degree of the dirtiness.