a detection sensor installed in a vehicle, the detection sensor configured to sense an obstacle located in a rear blind spot or a lateral blind spot of the vehicle;

a sequence setter configured, based on a sensing range of the detection sensor and multiple predetermined conditions, to set a sequence, among the multiple predetermined conditions, of determining whether the multiple predetermined conditions are satisfied; and

a warning determiner configured to sequentially determine, based on the sequence set by the sequence setter, whether the obstacle sensed by the detection sensor satisfies the multiple predetermined conditions and to determine, based on a result of determination with regard to satisfaction of the multiple predetermined conditions, whether the sensed obstacle is a target to be monitored,

wherein the sequence setter calculates a satisfaction level of each of the multiple predetermined conditions based on the sensing range of the detection sensor, and sets, based on the calculated satisfaction level, the sequence of determining whether the multiple predetermined conditions are satisfied.