Steering assist apparatus of vehicle and method for steering assist apparatus

A steering assist apparatus of a vehicle provided with a steering input apparatus that inputs a steering angle and a steering apparatus that steers wheels of the vehicle with a turning angle, the steering input apparatus and the steering apparatus performing a differential operation with a transmission ratio which is a ratio of a change amount of the steering angle to a change amount of the turning angle, the steering assist apparatus comprising:

• a steering drive that drives the steering apparatus;
• a processor configured to perform functions including:
• a steering control that executes an automatic steering mode in which the steering drive is controlled such that a turning angle is determined based on at least either a running state of the vehicle or road information to accomplish the turning angle;
• and
• a transmission ratio determination that determines, based on a turning curvature and at least either a running state of the vehicle or the road information, a first transmission ratio in the automatic steering mode to be a value different from a second transmission ratio in a manual steering mode in which the steering apparatus operates in accordance with the steering angle inputted via the steering input apparatus,
• wherein
• the transmission ratio determination changes the transmission ratio such that the larger the turning curvature, the larger the transmission ratio is.