1. A method of updating a pixel map, the method comprising: generating an X-ray transmission image by detecting an X-ray penetrating an object; determining whether a difference between a value associated with each one of the pixels forming the X-ray transmission image and a value associated with a nearby pixel adjacent to the each one of the pixels is equal to or greater than a predetermined reference value by comparing the value of the each one of the pixels with the value of the nearby pixel; determining a pixel having a value representing a difference equal to or greater than the reference value as a candidate for a bad pixel among the pixels if the difference is equal to or greater than the reference value; determining whether the candidate for the bad pixel is the bad pixel; and updating the pixel map in real time prior to performing image processing on the X-ray transmission image by use of the updated pixel map by showing the determined bad pixel on the pixel map if the candidate for the bad pixel is determined as the bad pixel.