An occupant protection auxiliary device includes: a seat mounted on a vehicle and on which an occupant is able to sit while facing a rear of the vehicle; a head detection sensor able to detect a relative position of a head of the occupant with respect to a headrest of the seat; and a control unit that determines whether to execute at least one of alerting the occupant, setting a speed limit, and changing a reclining angle based on a detection result of the head detection sensor.
