A vehicle driving assistant device, comprising: a sensor module; a camera module configured to photograph images; and a control module configured to: calculate a gradient value of a ground where a vehicle is located by the following Equation: gradient value=(first center axis value-second center axis value)/2, and determine 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, wherein the sensor module is mounted in a printed circuit board to be adjacent to the camera module and configured to measure a value of a center axis of the camera module, the sensor module is configured to measure the first center axis value when a front side of the vehicle is located in a first direction and the sensor module is configured to measure the second center axis value when the front side of the vehicle is located in a second direction which is opposite to the first direction, wherein the value of the center axis is a value indicating a ground direction among three axial directions, and wherein the center axis fixing error value includes a process error value obtained when the sensor module is warped at the time of being soldered to the printed circuit board and a mounting error value obtained when the sensor module is warped at the time of being mounted in the vehicle. a sensor module; a camera module configured to photograph images; and a control module configured to: calculate a gradient value of a ground where a vehicle is located by the following Equation: gradient value=(first center axis value-second center axis value)/2, and determine 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, calculate a gradient value of a ground where a vehicle is located by the following Equation: gradient value=(first center axis value-second center axis value)/2, and determine 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, wherein the sensor module is mounted in a printed circuit board to be adjacent to the camera module and configured to measure a value of a center axis of the camera module, the sensor module is configured to measure the first center axis value when a front side of the vehicle is located in a first direction and the sensor module is configured to measure the second center axis value when the front side of the vehicle is located in a second direction which is opposite to the first direction, wherein the value of the center axis is a value indicating a ground direction among three axial directions, and wherein the center axis fixing error value includes a process error value obtained when the sensor module is warped at the time of being soldered to the printed circuit board and a mounting error value obtained when the sensor module is warped at the time of being mounted in the vehicle.