In a driving lane deviation detection and warning system, a deviation detection ECU suppress a warning unit to supply a warning when an obstacle placed in a front of a vehicle is detected, and the driver's operation allows the vehicle to deviate from a driving lane. That is, when the driver's operation allows the vehicle to deviate from the driving lane in order to escape any collision with an obstacle, the deviation detection ECU instructs the warning unit to prevent the supply of a warning to the driver. This prevents the warning unit to provide an unnecessary warning to the driver even if the driver intends for the vehicle to deviate from the current driving lane of the vehicle. That is, it is possible to reliably instruct the warning unit to supply the warning to the driver when the vehicle deviates from the current driving lane against the driver's will.