Provided is an apparatus for sensing careless driving using a camera attached to a vehicle and warning a driver of a careless driving state, the apparatus including: a driving state determination unit for sensing a driving state of a vehicle based on a video of lanes; a lane-deviation sensing unit for sensing whether the vehicle deviated from the lane based on the video of lanes received from an image-processing unit; and a careless driving warning unit for generating a warning signal based on driving state information and lane deviation information received from the driving state determination unit and the lane-deviation sensing unit, respectively.