Provided is a vehicle driving assistant device, including: a sensor module; a camera module which photographs images; and a control module which calculates a gradient value of a ground where a vehicle is located based on a first center axis value measured by the sensor module when a front side of the vehicle is located in a first direction after being mounted and a second center axis value measured by the sensor module when the front side of the vehicle is located in a second direction which is opposite to the first direction and determines an effective area for following an object in the image based on a center axis fixing error value of the sensor module calculated using at least one of the first and second center axis values and the gradient value.