An apparatus for warning deviation from a lane has a deviation evaluation ECU 1. The deviation evaluation ECU 1 has a deviation evaluator 14 for performing an evaluation on a deviation by comparing a future position of a host vehicle with a lane future position. Before this deviation evaluation, a mask processor 13 determines whether a yaw angle of the host vehicle and a lateral position of the vehicle relative to a driving lane exceed their respective thresholds preliminarily determined. When either one or the both are not more than the thresholds, the mask processor 13 performs a mask process, without performing the deviation evaluation. A deviation evaluator 14 performs the deviation evaluation when the yaw angle and the lateral position relative to the driving lane both exceed their respective thresholds.