DRIVER MONITORING DEVICE, STORAGE MEDIUM STORING COMPUTER PROGRAM FOR DRIVER MONITORING, AND DRIVER MONITORING METHOD

A driver monitoring device has a processor configured to acquire number of passengers that are occupying a vehicle when the vehicle has begun to travel, to determine whether or not a driver is sitting in a driving seat based on an image near the driving seat of the vehicle which has been taken by an imaging unit and, when it has been determined that the driver is not seated in the driving seat, to determine that the driver is not seated in the driving seat if number of seated people, representing number of people that are sitting as detected by a seating sensor situated in each seat other than the driving seat of the vehicle, matches the number of passengers, and to determine that the driver is seated in the driving seat if the number of seated people is one less than the number of passengers.