To provide a drowsy driving prevention device capable of more effectively preventing drowsy driving.

A driver monitor specifies a contact address (mail address) of a driver on the basis of a face image of the driver photographed by a camera (S110), detects a level of drowsiness of the driver on the basis of the face image, and determines whether or not the level exceeds a threshold level (S115). If an affirmative determination is obtained (S115: Yes), a communication apparatus connected via an in-vehicle LAN is made to issue an electronic mail indicating an alarm on drowsy driving of the driver to the specified contact address (S120).