A driver monitoring device 1 comprises an imaging device 2 capturing an image of a face of a driver of a vehicle to generate an image, a face detecting part 31 detecting the face from the image, a face orientation estimating part 32 estimating a face orientation of the driver, and a cause judging part 33 judging a cause for the face not being detected. The face detecting part detects the face by extracting feature points of the face from the image. When the detection state of the face changes from “detected” to “not detected”, the cause judging part judges that the cause is an eating or drinking action of the driver if in a predetermined number of images until the detection state changes from “detected” to “not detected”, the face orientation is a front and the feature points at a lower region of the face are not extracted.