There is disclosed an image processing apparatus in which an optimum characteristic amount for use in an image processing can constantly correctly be extracted from an object image, an optimum image processing can therefore be performed, and a satisfactory image can be outputted. To achieve this, a first extraction section limits a predetermined area from the object image (X-ray image), and extracts a first characteristic amount from the predetermined area. A second extraction section extracts a second characteristic amount from a fixed area of the object image. A selection section selects a characteristic amount for use in a gray scale conversion processing from the first and second characteristic amounts based on a result of comparison of a difference between the first characteristic amount and the second characteristic amount with a predetermined threshold value.