In a cruising lane deviation detection system, when a boundary line is detected, a paint line drawn on a road surface to the left and right of an own vehicle is detected and a virtual line is set based on the position of a previously detected boundary line. The boundary line is set using at least either of the paint line and the virtual line. However, approach information indicating that the own vehicle is approaching a toll booth is acquired. When the approach information is acquired, setting of the virtual line as the boundary line is prohibited. Furthermore, whether or not there is risk of the own vehicle deviating from the boundary line is determined. When there is risk of deviation, an output that there is risk of deviation is performed. As a result, setting an erroneous virtual line as the boundary line can be suppressed.