An onboard monitoring device includes at least one image receiving device for receiving images coming from a rear of the vehicle, an image processing unit operateably connected to the at least one image receiving device to receive signals from the at least one image receiving device for analyzing the images so as to determine dimension change rate of the images based on velocities of the images, and a microprocessor operateably connected to the image processing unit so as to receive image signals from the image processing unit to determine locking status of the at least one vehicle door.