A steering assist apparatus is provided with a steering drive unit driving a steering apparatus, a steering control unit executing an automatic steering mode where a steering drive unit is controlled such that a turning angle is determined based on at least either a running state of the vehicle or road information to produce the determined turning angle, and a target differential angle control unit determining a target differential angle. The target differential angle control unit determines the target differential angle to be close to an automatic steering target differential angle used for the automatic steering mode, when input variation through a steering input apparatus is not detected during a transition period from a manual steering mode where the steering apparatus operates in response to the steering angle inputted through the steering input apparatus to the automatic steering mode or during an elapsed period after completing the transition period.