An autonomous driving assist device (100) comprises: a manual driving control unit (30) that controls a vehicle in accordance with the manual driving by a driver; an autonomous driving control unit (40) that uses the detection results of a first sensor (310, 315) that detects the driving state of the vehicle and the conditions around the vehicle, the autonomous driving control unit controlling autonomous driving; an abnormal state detection unit (10) that detects the presence of an abnormality in the first sensor; and a travel condition specification unit (20) that specifies a travel condition. When an abnormality in the first sensor is detected during control of autonomous driving, the autonomous driving control unit executes abnormal-period autonomous driving, in which the driving mode is changed in accordance with the specified travel condition, until a preset condition is satisfied. When the abnormal-period autonomous driving has ended, either the vehicle is stopped by the autonomous driving control unit, or control of manual driving is executed by the manual driving control unit.