A lane deviation alarm system is comprised of a forward-observed-point calculating section that calculates a forward observed point by multiplying a vehicle speed of a host vehicle and an anticipated deviation time; a forward-observed-point lateral-displacement calculating section that calculates a lateral displacement at the forward-observed-point, on the basis of a yaw angle and the forward-observed-point; a lane deviation tendency determining section that determines whether the host vehicle is in a lane deviation tendency, on the basis of the forward-observed-point lateral-displacement; and a criteria changing section that changes a criteria for determining a lane deviation tendency of the host vehicle, on the basis of a detecting condition of the lane defining line.