1. An image brightness control device to control brightness in real time, the device comprising: a preprocessing unit acquiring from a previous frame of an input image, dynamic range information corresponding to an image range of the input image using brightness values of color data of the input image and acquiring local brightness averages of local areas obtained by dividing the input image into the local areas having a predetermined size; and a tone mapping unit performing on a current frame of the input image, a local tone mapping operation on the local areas by comparing a bias applied value using a bias curve with a predicted average of a pixel in the input image predicted from local brightness values of the local areas, and not an entire brightness average of the input image, wherein the preprocessing unit includes: a dynamic range information acquiring unit setting a predetermined region of the brightness values of the pixels of the input image as an image range; and a local brightness average calculating unit dividing the input image to a plurality of the local areas and then calculating the local brightness averages which are averages of the brightness values of the pixels in the plurality of the local areas, wherein the dynamic range information acquiring unit includes: a histogram acquiring unit expressing the number of cases where the brightness values appear in the pixels of the input image as a histogram; a cumulative density function (CDF) calculating unit calculating a cumulative density function obtained by accumulating the histogram with respect to the brightness values; and an image range setting unit setting a predetermined area of the cumulative density function to the image range, and wherein the tone mapping unit includes: a bias curve applying unit acquiring the bias applied value by applying the brightness value of the pixel to the bias curve, wherein the bias applied value is less than the brightness value when the brightness value of the pixel is equal to or greater than a threshold value and is greater than the brightness value when the brightness value is less than the threshold value; a predicted average calculating unit calculating the predicted average from the local brightness averages by the use of a bi-linear interpolation method; and a stretching unit stretching the brightness value using a difference between the bias applied value and the predicted average.