A method of monitoring a blind spot of a monitoring vehicle by using a blind spot monitor is provided. The method includes steps of: the blind spot monitor (a) acquiring a feature map from rear video images, on condition that video images with reference vehicles in the blind spot are acquired, reference boxes for the reference vehicles are created, and the reference boxes are set as proposal boxes; (b) acquiring feature vectors for the proposal boxes on the feature map by pooling, inputting the feature vectors into a fully connected layer, acquiring classification and regression information; and (c) selecting proposal boxes by referring to the classification information, acquiring bounding boxes for the proposal boxes by using the regression information, determining the pose of the monitored vehicle corresponding to each of the bounding boxes, and determining whether a haphazard vehicle is located in the blind spot of the monitoring vehicle.